Separate cpp.Expression and ufl.Coefficient

[garth-wells]

From @blechta

Expressions have element associated with them. That enables any expression to be used immediately in integrals. This poses several problems:

1. Not all expressions are intended to be used in integrals. Why should they have element.
2. Attached element does not have any relevance apart of integrals. This is confusing
3. degree kwarg being shortcut for Lagrange element is overly confusing.

Proposal is to separate Expression from ufl.Coefficient in a following spirit

expression = Expression("x[0]*x[0]")
expression.eval(...)  # ok
u = interpolate(expression, some_function_space)  # ok
bc = DirichletBC(space, expression, where)  # ok
expression*dx  # failure: is not ufl expression

Expression(expression, element=...)*dx  # ok
create_pointwise_expression(expression, ...)*dx  # ok

The last two lines could be renamed to something more sensible, e.g.,

Coefficient(expression, element)*dx  # ok

See #2 for discussion about pointwise expressions.

[garth-wells]

See #183.