Split MappingCartesian::compute_fill into several functions.

Split MappingCartesian::compute_fill into several functions.

Presently MappingCartesian::compute_fill is called from the various
fill_fe*_values functions to compute quadrature points and normals.
compute_fill is called with dummy-normals in fill_fe_values and with
invalid face indices whenever face_index and subface_index has no
meaning. This makes the code hard to understand and the asserts that
check the arguments complicated. Split the compute_fill function into
several smaller functions to avoid passing around dummy or invalid
data.