feat: add multipole lens profile

[andreasfilipp]

adresses issue #139

coauthor: @RonanLegin

Multipole Gravitational Lensing Module

This Python module, part of a larger gravitational lensing library, provides functionality to simulate and analyze multipole gravitational lensing effects. It is designed to integrate seamlessly with PyTorch and offers detailed implementations for calculating deflection angles, lensing potentials, and convergence based on multipole expansions.

Features
Gravitational Lensing Calculations: Compute deflection angles, potentials, and convergence for multipole gravitational lenses.

Documentation
Refer to the docstrings within the code for detailed usage and examples. Equations used for the calculations are referenced from "Xu et al. 2014" (https://arxiv.org/pdf/1307.4220).

[andreasfilipp]

the convergence test for some reason does not pass. I do not know why

[ConnorStoneAstro]

Some minor changes, and one bigger change. Could you make it so that the user passes m as a tuple of which multipole orders they want? Then the individual functions can loop over that tuple and add the contributions from each multipole order.

Also can you add the multipole class to the test_lens_potential.py unit test? It doesn't look like its in there although I thought I saw you running that test

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.67%. Comparing base (c90b34b) to head (ab45b6f).
Report is 1 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #215      +/-   ##
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+ Coverage   93.52%   93.67%   +0.14%     
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  Files          61       63       +2     
  Lines        2859     2925      +66     
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+ Hits         2674     2740      +66     
  Misses        185      185              

Flag	Coverage Δ
unittests	93.67% <100.00%> (+0.14%)	⬆️

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