More cleanup in SSAFEM.

Now we use the same definition of effective viscosity in SSAFD and SSAFEM.
Document IceFlowLaw::effective_viscosity().
Fix doxygen warnings.
More cosmetic changes in SSAFEM.
Remove an outdated comment.
Use typedef to clean up the signature of a FEQuadrature method.
Improve some doxygen comments.
Whitespace cleanup.
Merge branch 'dev' into blatter