[FIX] Grid2D when range is not divisible by step

[mbeyeler]

Fixes the problem where the step size returned by Grid2D is slightly off, because the specified range is not divisible by step (issue #200)

[codecov-commenter]

Codecov Report

Merging #201 into master will increase coverage by 0.05%.
The diff coverage is 96.92%.

@@            Coverage Diff             @@
##           master     #201      +/-   ##
==========================================
+ Coverage   90.15%   90.21%   +0.05%     
==========================================
  Files          69       69              
  Lines        5658     5690      +32     
==========================================
+ Hits         5101     5133      +32     
  Misses        557      557              

Flag	Coverage Δ
#unittests	90.21% <96.92%> (+0.05%)	⬆️

Impacted Files	Coverage Δ
pulse2percept/percepts/base.py	96.22% <66.66%> (ø)
pulse2percept/utils/geometry.py	83.62% <88.88%> (+0.58%)	⬆️
pulse2percept/models/base.py	86.99% <100.00%> (ø)
pulse2percept/models/beyeler2019.py	94.11% <100.00%> (ø)
pulse2percept/models/tests/test_base.py	98.61% <100.00%> (ø)
pulse2percept/models/tests/test_beyeler2019.py	100.00% <100.00%> (ø)
pulse2percept/models/tests/test_nanduri2012.py	100.00% <100.00%> (ø)
pulse2percept/percepts/tests/test_base.py	100.00% <100.00%> (ø)
pulse2percept/utils/__init__.py	100.00% <100.00%> (ø)
pulse2percept/utils/tests/test_geometry.py	100.00% <100.00%> (ø)

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