Use optional dependencies

Solves the following issue(s):

Closes #364 (closed) and #367 (closed)

Core changes:

• Use optional dependencies for test, dev and doc. The command pip install . will just make a base installation for running code; unit testing can be enabled with pip install .[test]; usual development (testing + linting + formatting) is enabled by pip install .[dev]; building the doc is enabled by pip install .[doc].
• These can be also combined - for example: you get the full version (as until now) via pip install .[dev, doc].

Model-specific changes:

None

Documentation changes:

Use tabs in install doc.