Reduce number of dependencies

[MWieland]

We should try to further reduce the number of dependencies. In particular matplotlib and pillow are only used in one spot and should be substituted if possible.

[fwfichtner]

I agree, particularly because they are both only needed for the worldfile.

Generally we might also consider providing the option of only installing substets of the package, similar to Dask. We would only need to define our extras_require similar to how they do it. What do you think about that?

More information about setuptools. The only issue I see is that we don't have a core (defined yet).