split dependencies into optional groups

[tmchartrand]

I've been working on some code that uses basic functions of this library to parse the ABC atlas manifests etc - essentially only abc_atlas_access.abc_atlas_cache.manifest. I've been installing with --no-deps to avoid bringing in all the dependencies, but now I'm realizing that as I prepare to release this code there's no way for me to specify the abc_atlas_access dependency while overriding its dependencies in turn (short of depending on a fork instead).

I'd suggest at the very least splitting out the dependencies for code in src/ from those for notebooks (simpleITK for instance). My preferred solution would also split out anndata and cloud dependencies that are only needed for some of the core functionality, but I realize that could end up confusing some users.

Thanks, happy to discuss in person, just let me know!

[morriscb]

Hey @tmchartrand,

Thanks for the suggestion. I've been wanting to reorganize the project file a bit and this is a good enough reason to do so. I'm working on a release that is going out with the aging mouse dataset so I'll see if I can't separate dependencies out for the notebooks and testing.

[morriscb]

#67