[ENH] Add generic rectangle implant and remove codecov

[jgranley]

Description

This adds a generic rectangle implant, which has been used previously in multiple papers.

Currently if you want to make a custom implant, you have to make a custom electrode grid, then create a prosthesis system subclass wrapping this earray, which is cumbersome, and lacks some of the basic functions that premade implants have.

This adds a generic p2p.implants.RectangleImplant which does all this for the user, so they can have an implant with a custom location, shape, spacing, electrode type, etc with one line.

It defaults to the 15x15 implant used in Granley et al 2022 and Granley et al 2023

This also removes codecov (see #632 )

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation change

[codecov]

Codecov Report

Attention: Patch coverage is 93.42105% with 5 lines in your changes missing coverage. Please review.

Project coverage is 88.01%. Comparing base (7a9da85) to head (bcf8fa6).
Report is 142 commits behind head on master.

❗ Current head bcf8fa6 differs from pull request most recent head b3dbf94

Please upload reports for the commit b3dbf94 to get more accurate results.

Files	Patch %	Lines
pulse2percept/implants/base.py	83.33%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #631      +/-   ##
==========================================
- Coverage   93.83%   88.01%   -5.82%     
==========================================
  Files          90      128      +38     
  Lines        7782    11556    +3774     
==========================================
+ Hits         7302    10171    +2869     
- Misses        480     1385     +905     

Flag	Coverage Δ
unittests	88.01% <93.42%> (-5.82%)	⬇️

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