FETools::get_fe_from_name() implemented for nonzero codimension

In addition to the new functionality, the internal structures mapping names to finite element factories have been improved, thus avoiding several specializations of functions.

This pull request fixes #86 (issue)
This is here as a reminder to us that all these types of FiniteElements are still not codimension compatible, in the sense that their implementation is either incomplete for codimension one or two, or their instantiation is missing. 

I'll open an issue with this list, but I did not want to delete them from here, since the test should succeed once implementation/instantiations are fixed.