SDG import scripts (datacommonsorg#850)

* add input files

* add svs

* add scripts for generating svs

* fix

* remove extra commas

* add script for pv schema

* generate new enums for each property

* update schema

* fix

* fix

* add schemaful svs

* add city maps

* update

* cities

* preprocess

* updates

* fix

* fix

* USE SUBMODULE

* DELETE old scripts/files and update cities

* update cities

* some updates to test

* delete file

* update tests

* add csv to lfs

* update

* fix

* update modules

* modules

* tests

* test

* test

* lint

* tests

* lint

* strings

* remove nan series

* update variable codes

* address comments

* updates

Author: n-h-diaz

.gitmodules

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+[submodule "scripts/un/sdg/sdg-dataset"]
+	path = scripts/un/sdg/sdg-dataset
+	url = https://code.officialstatistics.org/undata2/data-commons/sdg-dataset.git

scripts/un/sdg/.gitattributes

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+csv/* filter=lfs diff=lfs merge=lfs -text
+dc_generated/* filter=lfs diff=lfs merge=lfs -text

scripts/un/sdg/README.md

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+# UN Stats Sustainable Development Goals
+
+This import includes country-level data from the [UN SDG Global Database](https://unstats.un.org/sdgs/dataportal). Data is read from the submodule `sdg-dataset` which is managed by UN Stats.
+
+
+To generate city dcids:
+```
+python3 cities.py <DATACOMMONS_API_KEY>
+```
+(Note: many of these cities will require manual curation, so this script likely should not be rerun.)
+
+To process data and generate artifacts:
+```
+python3 process.py
+```
+Produces:
+* schema/ folder: 
+  * measurement_method.mcf
+  * schema.mcf (classes and enums)
+  * sdg.textproto (vertical spec)
+  * series.mcf (series mcf)
+  * sv.mcf
+  * unit.mcf
+* csv/ folder: 
+  * [CODE].csv
+(Note that the `schema/` folder is not included in the repository but can be regenerated by running the script.)
+
+To run unit tests: 
+```
+python3 -m unittest discover -v -s ../ -p "*_test.py"
+```