This restructuring allows for easier separation of modules into
separate repositories later. Also, now pip install with repo url can be
used.

The setup.py files have also been updated to correctly reference each
other. Module versions are not extracted from git state

- if a field access is printed in compass or index notation seems to
  depend on the sympy version
- removed doctests that rely on the printed form

- uses ast instead of text-based editing
- can handle Piecewise defined functions
- new decorator for simplified usage