Vectorise evaluations in Python Expressions and SubDomains

[Prof Garth Wells]

Call-backs from C++ to user-defined Python functions for Expression and SudDomain are slow. It could be made much faster by supporting NumPy 'vectorisation'.

This shouldn't be very difficult once we've switched to pybind11 and using Eigen objects.

[Nico Schlömer]

Issue #1011 was marked as a duplicate of this issue.