Add gravitational lensing introduction

[ConnorStoneAstro]

A jupyter notebook which explains the basics of gravitational lensing, while using caustics for visual demos

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.31%. Comparing base (41a2f57) to head (611b74a).
Report is 2 commits behind head on dev.

Additional details and impacted files

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  Coverage   90.31%   90.31%           
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Flag	Coverage Δ
unittests	90.31% <ø> (ø)

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[NolanSmyth]

Such a cool tutorial. Great information and also awesome way to show off a variety of features in caustics