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)
I don't understand this comment. You don't need to derive from Subscriptor to achieve this -- you only need to have one virtual function (which this class has) to enable dynamic_cast. There may be other good reasons to use Subscriptor, but this comment doesn't capture it. (As a side note, comments that describe "how it happened" -- here: "...was introduced..." -- are usually inefficient because people don't care how we got there as a path in time; they care about what a function or class does currently, so comments should state facts and rationale, not history.)