[ENH] Add PRIMA prototypes

[mbeyeler]

• Add the different PRIMA generations
  • PRIMA: 100um, used in clinical trial
  • PRIMA75: 75um pixel pitch
  • PRIMA55: 55um pixel pitch
  • PRIMA40: 40um pixel pitch
• HexElectrode, PhotovoltaicPixel
• Add references
• Gallery example

[codecov-commenter]

Codecov Report

Merging #188 into master will increase coverage by 0.83%.
The diff coverage is 95.84%.

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   89.16%   90.00%   +0.83%     
==========================================
  Files          69       69              
  Lines        5401     5620     +219     
==========================================
+ Hits         4816     5058     +242     
+ Misses        585      562      -23     

Flag	Coverage Δ
#unittests	90.00% <95.84%> (+0.83%)	⬆️

Impacted Files	Coverage Δ
pulse2percept/implants/tests/test_argus.py	100.00% <ø> (ø)
pulse2percept/viz/__init__.py	100.00% <ø> (ø)
pulse2percept/implants/electrodes.py	90.65% <85.36%> (+2.81%)	⬆️
pulse2percept/implants/prima.py	94.49% <93.33%> (+33.38%)	⬆️
pulse2percept/implants/base.py	97.97% <93.93%> (-2.03%)	⬇️
pulse2percept/__init__.py	100.00% <100.00%> (ø)
pulse2percept/implants/__init__.py	100.00% <100.00%> (ø)
pulse2percept/implants/bva.py	91.17% <100.00%> (+4.33%)	⬆️
pulse2percept/implants/electrode_arrays.py	95.97% <100.00%> (+0.93%)	⬆️
pulse2percept/implants/tests/test_base.py	100.00% <100.00%> (ø)
... and 6 more

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