Quickfix for enabling the notation f*dx(..., degree=3, rule=...).

Just routes the arguments to metadata["quadrature_degree"],
metadata["quadrature_rule"]. Later we can make them easier accessible.
Add function coordinate_element() to Domain.
Use domain.coordinate_element() instead of domain.coordinates()
wherever possible without any major rewriting.
Merge branch 'martinal/topic-quickfix-dx-keywords-degree-and-rule'