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)
Catch 22. What about a new function `get_fe_by_name<dim,spacedim>()` called by the old one, with the old one deprecated?