Add vmin and vmax to ImageGrid and related functions

[SarthakJariwala]

Images visualized using ImageGrid can now be compared against each other using the same colorbar min-max limits -

This is done via vmin and vmax parameters in ImageGrid

g = isns.ImageGrid(cells, vmin=0, vmax=1)

You can also use specify individual vmin/vmax for a collection of images:

g = isns.ImageGrid([img1, img2, img3], vmin=[10, 20, 30], vmax=[200, 210, 220])

See #237

[codecov-commenter]

Codecov Report

Merging #287 (416a6b8) into master (4d54282) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
+ Coverage   96.89%   96.94%   +0.04%     
==========================================
  Files           9        9              
  Lines         773      785      +12     
  Branches      202      204       +2     
==========================================
+ Hits          749      761      +12     
  Misses         11       11              
  Partials       13       13              

Impacted Files	Coverage Δ
src/seaborn_image/_grid.py	98.57% <100.00%> (+0.05%)	⬆️

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