Improve wrapping Special methods

[jpfeuffer]

• Contribute to Cython and allow parsing of diphtong operators "+"=, "-=", ...: “Overloading operator '+=' not yet supported.” cython/cython#3696 and finish Fixed in-place operators "+=" and "-=". cython/cython#3700
• Implement wrapping of ALL special methods: https://cython.readthedocs.io/en/latest/src/userguide/special_methods.html#special-methods. Slicing would be interesting. But also unary operators.
• Check if using the single special comparison functions is better than "richcmp" (which is what we are using right now. It might be that it was the only options back then).
• Allow overloading special methods and let them return/accept other types. Currently the special methods, like operators and to some extend the comparators are limited to only accepting the same type and returning the same type. This might not always be the same case. See e.g. Uncomment wrapping of other operators in Empirical Formula OpenMS#5915