Refactored project setup of optional fe2vtu module.

Follow-up on !31 (merged).

If you want to have the feflow converter install the optional dependency group feflow:

pip install .[dev,test,feflow]
# OR
make setup
pip install .[feflow]

Later on (when released via PyPI) users need to install it via:

pip install ogstools[feflow]

Also sphinx-apidoc is now called behind the scenes via its sphinx extension (was explicitly called in docs Makefile).