Deprecate and remove DirichletBC handling via magic mesh file info

[Prof Garth Wells]

The code in https://bitbucket.org/fenics-project/dolfin/src/e3cee8cb78ff7e27e4d52d7cb223a0492d9a7383/demo/documented/bcs/python/demo_bcs.py?at=master&fileviewer=file-view-default is cryptic in that it relies on data that is silently extracted from a mesh file.

It was agreed some time ago that data extraction should be explicit, so this interface should be deprecated and then removed.

It is also a maintenance burden in parallel, and will ease the pybind11 transition.