[ENH] Neuropythy doc and error

[jgranley]

Adds error when using neuropythy classes without it being installed, and updated doc with installation instructions

[codecov]

Codecov Report

Attention: 484 lines in your changes are missing coverage. Please review.

Comparison is base (7a9da85) 93.83% compared to head (64d4c70) 92.18%.
Report is 139 commits behind head on master.

Files	Patch %	Lines
pulse2percept/topography/base.py	69.44%	99 Missing ⚠️
pulse2percept/implants/cortex/neuralink.py	53.37%	83 Missing ⚠️
pulse2percept/topography/cortex.py	79.81%	43 Missing ⚠️
pulse2percept/datasets/han2021.py	36.00%	32 Missing ⚠️
pulse2percept/implants/ensemble.py	69.44%	22 Missing ⚠️
pulse2percept/models/granley2021.py	88.53%	18 Missing ⚠️
pulse2percept/datasets/tests/test_han2021.py	37.03%	17 Missing ⚠️
pulse2percept/models/cortex/base.py	79.48%	16 Missing ⚠️
pulse2percept/models/base.py	82.71%	14 Missing ⚠️
pulse2percept/stimuli/videos.py	84.50%	11 Missing ⚠️
... and 32 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #608      +/-   ##
==========================================
- Coverage   93.83%   92.18%   -1.65%     
==========================================
  Files          90      128      +38     
  Lines        7782    11483    +3701     
==========================================
+ Hits         7302    10586    +3284     
- Misses        480      897     +417     

Flag	Coverage Δ
unittests	92.18% <89.20%> (-1.65%)	⬇️

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