Merge branch 'martinal/topic-add-sobolev-spaces-to-elementlist' into future_master
Merge branch 'martinal/topic-optimize-indexed-operands' into future_master
Work on cleanup of geometry quantities.
Move some comments around.
Rename ReferenceCoordinate -> CellCoordinate.
Rename reference_coordinate -> cell_coordinate.
Rename ReferenceFacetCoordinate -> FacetCoordinate.
Rename reference_facet_coordinate -> facet_coordinate.
Rename CellOriginCoordinate -> PhysicalCellOrigin.
Rename PhysicalCellOrigin -> PhysicalCellOrigo.
Rename ReferenceFacetJacobian -> CellFacetJacobian. (and some more)

Accidentally did this rename before commiting the following:
- Move some classes around in geometry.py to group better
- Add sketch of CellTangents, FacetTangents types (disabled)
- Rename barycenter -> midpoint (still disabled types)
Rename FacetJacobian -> PhysicalFacetJacobian.
Rename facet_jacobian -> physical_facet_jacobian.
Drop plan for renaming Jacobian->PhysicalCellJacobian, because "The Jacobian" is a central well known concept in this context.
Rename PhysicalCellOrigo -> CellOrigo.
Rename PhysicalFacetOrigo -> FacetOrigo.
Rename PhysicalFacetJacobian -> FacetJacobian.
Rename physical_facet_jacobian -> facet_jacobian.
Remove geometry classes from the ufl.* namespace that are intended for
internal usage. Document the geometry classes a bit better.
Import some internal geometry classes in test.