WIP: Add 2D (polar) maps of structure functions

[BrodiePearson]

This PR will allow the creation of 2D polar maps of structure functions, showing how the functions vary with both separation distance and separation direction.

This generalizes the current software functionality, which can diagnose the effects of separation distance in the directions of the data-grid.

• Core functionality for 2D maps - @BrodiePearson
• Tests for 2D maps - @cassidymwagner
• Example of 2D map calculation - @BrodiePearson
• Documentation - @BrodiePearson

[codecov]

Codecov Report

Attention: Patch coverage is 20.58824% with 108 lines in your changes missing coverage. Please review.

Project coverage is 69.61%. Comparing base (3acfbdf) to head (39a36cf).
Report is 4 commits behind head on main.

Files	Patch %	Lines
fluidsf/generate_2D_SF_maps.py	11.42%	62 Missing ⚠️
fluidsf/calculate_2D_SF_maps.py	8.57%	32 Missing ⚠️
fluidsf/xy_shift_array.py	12.50%	14 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #22       +/-   ##
===========================================
- Coverage   99.13%   69.61%   -29.53%     
===========================================
  Files           8       11        +3     
  Lines         232      362      +130     
===========================================
+ Hits          230      252       +22     
- Misses          2      110      +108     

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[BrodiePearson]

@cassidymwagner I think this PR's code/example is ready for review now! I still need to add all the other SFs of interest, as it currently just does advective SFs, but I think that addition should be pretty quick.

I added an example, which requires the 2layer data set (only uses the final snapshot) and includes:

1. A validation of its zonal and meridional components against the existing 1-direction SF calculations
2. A 2D map of the advective structure functions from the final snapshot
3. A plot of angles and separation distance to visualize how they vary in two dimensions

The plots created are:

[cassidymwagner]

I'm going to merge this as well since the features are ready and any additional documentation can be worked on in #23. I'll make a new branch for testing coverage soon to handle testing for all the modules.