Merge esa-apex-toolbox-python repo

[soxofaan]

Under #3 I created a new repo https://github.com/ESA-APEx/esa-apex-toolbox-python

But while working on #4 I started to realize that there is a lot of common ground between ESA-APEx/apex_algorithms and ESA-APEx/esa-apex-toolbox-python, e.g. the schemas of resources, how to parse/validate them, ...

At the moment there seems to be emerging some kind of bi-directional dependency between these projects and I think it makes more sense (development/maintenance-wise) to just merge esa-apex-toolbox-python as a subproject into apex_algorithms. Especially at this early stage of development when we're not yet publishing stable packages, which makes cross-dependency management hard.

[soxofaan]

As discussed over at ESA-APEx/apex_documentation#69 we're now having broken builds because of hidden and now broken coupling between these projects