Add human-readable PSR shorthand codes (solar, gas, nuclear, etc.)

Users can now query with readable shorthands instead of ENTSO-E codes:
  psr_type="solar" | "wind_onshore" | "gas" | "nuclear" | ...

All three formats work: "B16", "Solar", "solar".
Expose PSR_CODES and COUNTRY_NAMES dicts for discoverability.

Author: jsulopzs

examples/balancing.ipynb

@@ -2,7 +2,7 @@
  "cells": [
   {
    "cell_type": "markdown",
-   "id": "c02a18eb",
+   "id": "05b5715a",
    "metadata": {},
    "source": [
     "# Balancing Data — ENTSO-E Examples\n",
@@ -13,13 +13,13 @@
   {
    "cell_type": "code",
    "execution_count": 1,
-   "id": "02e5427b",
+   "id": "d14a42fa",
    "metadata": {
     "execution": {
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-     "iopub.status.idle": "2026-02-15T15:15:56.023878Z",
-     "shell.execute_reply": "2026-02-15T15:15:56.023430Z"
+     "iopub.execute_input": "2026-02-15T15:20:54.919400Z",
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+     "shell.execute_reply": "2026-02-15T15:20:55.341934Z"
     }
    },
    "outputs": [],
@@ -34,7 +34,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "18cb04fe",
+   "id": "f1ceeb12",
    "metadata": {},
    "source": [
     "## 1. Imbalance Prices — France (1 week)"
@@ -43,13 +43,13 @@
   {
    "cell_type": "code",
    "execution_count": 2,
-   "id": "b69608c6",
+   "id": "de4af948",
    "metadata": {
     "execution": {
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-     "iopub.status.idle": "2026-02-15T15:15:57.900759Z",
-     "shell.execute_reply": "2026-02-15T15:15:57.899957Z"
+     "iopub.execute_input": "2026-02-15T15:20:55.344719Z",
+     "iopub.status.busy": "2026-02-15T15:20:55.344564Z",
+     "iopub.status.idle": "2026-02-15T15:20:56.789653Z",
+     "shell.execute_reply": "2026-02-15T15:20:56.788722Z"
     }
    },
    "outputs": [
@@ -145,13 +145,13 @@
   {
    "cell_type": "code",
    "execution_count": 3,
-   "id": "dd580a6c",
+   "id": "6938eb36",
    "metadata": {
     "execution": {
-     "iopub.execute_input": "2026-02-15T15:15:57.908139Z",
-     "iopub.status.busy": "2026-02-15T15:15:57.908003Z",
-     "iopub.status.idle": "2026-02-15T15:15:58.677004Z",
-     "shell.execute_reply": "2026-02-15T15:15:58.676528Z"
+     "iopub.execute_input": "2026-02-15T15:20:56.796779Z",
+     "iopub.status.busy": "2026-02-15T15:20:56.796603Z",
+     "iopub.status.idle": "2026-02-15T15:20:57.511047Z",
+     "shell.execute_reply": "2026-02-15T15:20:57.510382Z"
     }
    },
    "outputs": [
@@ -1678,7 +1678,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "5f7bc13a",
+   "id": "79baf71f",
    "metadata": {},
    "source": [
     "## 2. Imbalance Prices — Multi-Country"
@@ -1687,13 +1687,13 @@
   {
    "cell_type": "code",
    "execution_count": 4,
-   "id": "312213f6",
+   "id": "edfbbb7d",
    "metadata": {
     "execution": {
-     "iopub.execute_input": "2026-02-15T15:15:58.679064Z",
-     "iopub.status.busy": "2026-02-15T15:15:58.678926Z",
-     "iopub.status.idle": "2026-02-15T15:16:01.597170Z",
-     "shell.execute_reply": "2026-02-15T15:16:01.596139Z"
+     "iopub.execute_input": "2026-02-15T15:20:57.513853Z",
+     "iopub.status.busy": "2026-02-15T15:20:57.513663Z",
+     "iopub.status.idle": "2026-02-15T15:20:59.304609Z",
+     "shell.execute_reply": "2026-02-15T15:20:59.303382Z"
     }
    },
    "outputs": [
@@ -5781,7 +5781,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "2deca6a7",
+   "id": "f956d7fa",
    "metadata": {},
    "source": [
     "## 3. Price Distribution — Histogram"
@@ -5790,13 +5790,13 @@
   {
    "cell_type": "code",
    "execution_count": 5,
-   "id": "3d8946c8",
+   "id": "8ac2986c",
    "metadata": {
     "execution": {
-     "iopub.execute_input": "2026-02-15T15:16:01.605247Z",
-     "iopub.status.busy": "2026-02-15T15:16:01.605008Z",
-     "iopub.status.idle": "2026-02-15T15:16:01.627617Z",
-     "shell.execute_reply": "2026-02-15T15:16:01.627123Z"
+     "iopub.execute_input": "2026-02-15T15:20:59.311871Z",
+     "iopub.status.busy": "2026-02-15T15:20:59.311679Z",
+     "iopub.status.idle": "2026-02-15T15:20:59.333425Z",
+     "shell.execute_reply": "2026-02-15T15:20:59.332967Z"
     }
    },
    "outputs": [
@@ -6703,7 +6703,7 @@
   },
   {
    "cell_type": "markdown",
-   "id": "45d08e70",
+   "id": "bfc4b67a",
    "metadata": {},
    "source": [
     "## 4. Imbalance Prices vs Day-Ahead Prices"
@@ -6712,13 +6712,13 @@
   {
    "cell_type": "code",
    "execution_count": 6,
-   "id": "9ccdac10",
+   "id": "0ce2269d",
    "metadata": {
     "execution": {
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-     "iopub.status.idle": "2026-02-15T15:16:02.653767Z",
-     "shell.execute_reply": "2026-02-15T15:16:02.653230Z"
+     "iopub.execute_input": "2026-02-15T15:20:59.334974Z",
+     "iopub.status.busy": "2026-02-15T15:20:59.334858Z",
+     "iopub.status.idle": "2026-02-15T15:21:00.145923Z",
+     "shell.execute_reply": "2026-02-15T15:21:00.145290Z"
     }
    },
    "outputs": [