add_subexpressions_for_constants and new_filtered fix

Two small patches:

• Added a function add_subexpressions_for_constants to pystencils.simp. This function extracts numerical constants like 2 or 1/3 from equations. This is helpful because SymPy does not exclude common factors from sums if they are not symbols. Excluding them this way can reduce the number of multiplications. In some cases, additional common subexpressions can also be found.
• Changed AssignmentCollection.new_filtered to use self.copy to preserve other members of the assignment collection.