Clean extended SymPy functions

There are some functions which extend a SymPy function. These should all be implemented in a single module/file

added refactor label

changed milestone to %Release 1.0

changed milestone to %Release 1.1

Move functions.py, fast_approximations.py, sympyextensions.py, integer_functions.py, (placeholder_functions.py) to one sympy_extensions module.

mentioned in work item #87

changed milestone to %Release 2.0

mentioned in task #87