Changes to support new Domain representation in UFL.

Some of the commit messages before squashing:
- avoid getting ufl.Measure object from ufl.Integral.
- some refactoring to clarify common metadata extraction.
- refactoring to get rid of cell() usage as much as possible,
  also trying to make limitations of current cell() usage clear
  where it is used.
- avoid getting tdim and other domain information from form_data,
  using itg_data instead for future support for differing domains.
- removing common_cell and element_mapping arguments from form data computation.
- remove cell signature in jit object, this is part of form signature.
- make use of domain explicit in errorcontrol code.
- Use new ufl_element.reconstruction_signature() instead of repr.
- Update reference data pointer. Only signatures changed.
Update reference data pointer to badcbe6c6769b39915d2cbc7adebd72dd27dc838.
Merge branch 'martinal/topic-multidomain-infrastructure'

Conflicts:
	test/regression/ffc-reference-data-id