This yields better error messages

move_constants_before_loop needed to be changed to move already-existing variables up if necesesary and to preserve constness when renaming variables

also add a test for subexpressions

It takes a list of assignments to staggered fields and otherwise behaves like create_kernel.
The old create_staggered_kernel was not removed because all the optimizations still need to be ported over.

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

- split groups have been ignored

- bugfix in vector-typing of piecewise functions
- cast_function is now a sympy atom - fixes problems with sympy > 1.1
- replace_inner_stride_with_one is a bit faster now

- support aligned load/stores
- nontemporal stores
- aligned memory allocation for arrays and temporary buffers

- SSA form is checked
- loop independence condition is checked
- bug fix in Field.create_generic when using index_shape