[FIX] Grid2D plot when step is a tuple

[mbeyeler]

This PR fixes issue #442, when step of a Grid2D object is a tuple:

p2p.utils.Grid2D((-5, 5), (-5, 5), step=(0.5, 1)).plot(style='cell')

[codecov-commenter]

Codecov Report

Merging #446 (fe952b8) into master (7a9da85) will increase coverage by 0.10%.
The diff coverage is 95.66%.

@@            Coverage Diff             @@
##           master     #446      +/-   ##
==========================================
+ Coverage   93.83%   93.93%   +0.10%     
==========================================
  Files          90       90              
  Lines        7782     7868      +86     
==========================================
+ Hits         7302     7391      +89     
+ Misses        480      477       -3     

Flag	Coverage Δ
unittests	93.93% <95.66%> (+0.10%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pulse2percept/viz/tests/test_base.py	100.00% <ø> (ø)
pulse2percept/utils/images.py	91.54% <50.00%> (-1.21%)	⬇️
pulse2percept/implants/base.py	94.95% <82.14%> (-2.96%)	⬇️
pulse2percept/models/base.py	87.64% <83.33%> (+0.28%)	⬆️
pulse2percept/viz/base.py	88.67% <90.90%> (-0.69%)	⬇️
pulse2percept/utils/geometry.py	90.17% <91.52%> (+3.59%)	⬆️
pulse2percept/models/granley2021.py	93.64% <94.44%> (+0.78%)	⬆️
pulse2percept/models/beyeler2019.py	95.66% <95.00%> (-0.16%)	⬇️
pulse2percept/implants/bvt.py	90.90% <100.00%> (-0.99%)	⬇️
pulse2percept/implants/electrode_arrays.py	95.15% <100.00%> (+0.32%)	⬆️
... and 14 more

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