Map multiple functions to images in ImageGrid

[SarthakJariwala]

ImageGrid gains the ability to map multiple functions to the input image/s. Overall, there are 4 use cases :

• List of map_func mapped to a List of 2D images

g = isns.ImageGrid([img1, img2, img3], map_func=[f1, f2, f3])

• List of map_func applied to a single 2D image

g = isns.ImageGrid(img_2d, map_func=[f1, f2, f3])

• Single map_func applied to a list of 2D images

g = isns.ImageGrid([img1, img2, img3], map_func=f3)

• Single map_func applied to a 3D image

g = isns.ImageGrid(img_3d, map_func=f1)

Parameters for individual functions can also be passed as a Dict or List of Dicts to map_func_kw parameter in ImageGrid

For eg -

from skimage.filters import gaussian, hessian, median

g = isns.ImageGrid(
    [img1, img2],
    map_func=[gaussian, median, hessian],
    map_func_kw=[{"sigma" : 2}, None, {"mode" : "reflect"}]
)

This PR also adds a new dataset retina-gray for documentation purposes.

retina = isns.load_image("retina-gray")

[codecov-commenter]

Codecov Report

Merging #236 (ee73cc8) into master (787122f) will increase coverage by 1.65%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #236      +/-   ##
==========================================
+ Coverage   95.24%   96.89%   +1.65%     
==========================================
  Files           9        9              
  Lines         715      773      +58     
  Branches      171      202      +31     
==========================================
+ Hits          681      749      +68     
+ Misses         17       11       -6     
+ Partials       17       13       -4     

Impacted Files	Coverage Δ
src/seaborn_image/_datasets.py	100.00% <100.00%> (ø)
src/seaborn_image/_grid.py	98.52% <100.00%> (+3.83%)	⬆️

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