Summand and Module clean up part 1

[hschreiber]

I extracted Summand and Module from approximation.h to clean it up a bit and also remove unused methods. But I would also like to replace all nested vector outputs with flat vector outputs, to avoid copies at cython level (when possible). But before doing so, I would like to have the new changes validated as it is already a lot.

So @DavidLapous, I would like you to look at the following points:

• verify all modifications work also on all your own notebooks etc. I only tested the pytests and a few public notebooks.
• verify if I didn't removed a method you planned to use later
• tell me if I can completely erase all python code I commented or if you want to find alternatives to the MultiDiagram* classes (we decided to get rid of) first.

I also added set_target_properties(${target_name} PROPERTIES COMPILE_FLAGS "--no-warnings") for all external libraries, to find more easily compilation problems when they occur.