Deprecate Expression as base class for Python classes in favour of UserExpression

[Prof Garth Wells]

The Python code for Expression is extremely complicated because it does too many things. See #883 and https://bitbucket.org/fenics-project/dolfin/pull-requests/265.

To simplify the code, in the new pybind11 interface user-defined sub-classes of dolfin::Expression implemented in Python should be subclasses of UserExpression rather than Expression, i.e.

class MyExpreession(Expression):
    pass

becomes

class MyExpreession(UserExpression):
    pass

Add an alias to next release (UserExpression = Expression) and a add a deprecation warning that the base class needs to be changed.

[Prof Garth Wells]

I think a deprecation mechanism for this is a bit complicated in SWIG. Probably best handled via a the 2018.1 release notes.

[Chris Richardson]

@blechta - do you have any idea how to do this?

[Jan Blechta]

As Garth said above, I guess.

[Jan Blechta]

Fix issue #934 [skip ci]

→ <<cset 07f8e6c5df73>>