Start working on changing Argument count to number.
Implement is_cellwise_constant for geometric quantities on nonaffine domains.
Extend tests to cover coefficients and arguments a bit.
Various small fixes related to is_cellwise_constant and non-affine domains.
Change book snippets tests to take Argument number. This is a little used feature, accepting the interface breaking.
Add number to Argument constructor calls in tests.
Complete most of the work changing Argument(V,count) to Argument(V,number,part).

The current implementations of form operators rhs,lhs,system,action,
and a few others will not deal with arguments with parts at the
moment. It might be a nontrivial task to generalize them.
Merge branch 'master' into martinal/topic-replace-argument-count-with-number-and-block

Conflicts:
	ufl/algorithms/preprocess.py
Update changelog.
Add missing deprecate import.
Minor bugfix in preprocess_expression after the last merge.