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)
Yes, I thought of a tuple of tuple. You'd get the elements using
```
  table_of_maps.get<dim>().get<spacedim>()
```
But I'm ok with the current approach. It's localized to the internals and does not leak to user space.