Refactor preprocessing of forms and expressions

[Martin Sandve Alnæs]

Currently we have preprocess(form, ...) and preprocess_expression(expr, ...) which is used only by uflacs, but they both do mostly the same and can be merged with some refactoring. At the same time, the form signature should be computed from the form resulting from a canonical merging of integrals over subdomains. Currently this merging happens in the translation to integral_data and is thus not seen in preprocessed_form.

Work items:

• Get rid of replacement of functions by using function mappings in ffc: TODO
• Split integrals by subdomain id before signature is computed: TODO
• Reuse preprocessing code between form and expression preprocess: TODO

[Martin Sandve Alnæs]

Superseded by https://bitbucket.org/fenics-project/ufl/issue/36/refactor-preprocessing-and-its-use-in-ffc