API

This PR updates the API documentation using numpydoc and makes the overall __init__ structure a little more pythonic for use with sphinx.ext.autosummary. I think this should make the API documentation much more readable. I would also update the API to something like this:

import pyslk

pyslk.list('/arch')
pyslk.retrieve('/arch/test.txt')

I think this is also a little more pythonic than having to import submodules explicitly. The namespace pyslk also makes clear that we use slk, so i renamed the function during import in __init__ to remove the slk prefix. Note, that also the old API should still work...

Using sphinx.ext.autosummary gives a nice overview: