Clean up and Bug Fixes

Fixes #38 (closed)

Further changes:

• Third party warnings are suppressed now as they just polluted the warning summary in pystencils.
• Removing the math optimisations as they are not used in pystencils. An Issue is created to reintroduce them again in a clean way in the future
• Removing the joblib workaround as joblib is a hard dependency of pystencils
• Fixing SymPy 1.9 as maximum version
• Removing sympy.multiply_elementwise due to https://github.com/sympy/sympy/issues/22353. A workaround with NumPy is introduced which seems to be way more robust because NumPy has a more static API
• Introducing more extra requires, which are needed to have all features of pystencils
• Minor cleanup of some test cases and the datahandling