Maintenance: split ndinterp long __init__.py file functions into separate files

[GenevieveBuckley]

Closes #415

The __init__.py file of the dask_image.ndinterp has become very large (~780 lines), and currently contains five public API functions, plus associated helper functions. This is not ideal, since bigger files are harder to understand and maintain, and if anyone else adds a new function to the ndinterp package it'll push the file size over what's generally considered best practice (pylint suggests a maximum line limit of around 1000)

To avoid future problems, in this PR I've split the ndinterp init.py file into separate smaller files while keeping the git history preserved for each line. Now you can more easily see which part of the code is the most relevant, and each file is a more manageable size (roughly150-250 lines).

I followed this guide to split the files while still preserving the full git history.

[GenevieveBuckley]

fyi I checked the git history was kept by looking at the GitLens inline display of the git blame info in VScode. It seems to have worked properly. I'll cross check it against the inline git blame info shown in the github web browser.

[m-albert]

This is great!