Use mypy for static type checking

[pkienzle]

Sasmodels has type annotation for many functions. Mypy static type checking should be run as part of the test suite on every check-in.

Mypy should also be run on sasview, bumps and periodictable, with type signatures gradually added to all the functions.

Note: the NCNR nice instrument control project (NCNR internal) has type annotations defined for h5py, numpy, matplotlib, bz2 and cProfile that can used as a starting point if these projects do not yet provide their own.