Allow querying whether a DoF is located on a vertex, line, etc.

In response to https://groups.google.com/forum/#!searchin/dealii/query$20type/dealii/S3peBCKPUQM/AcyGarA7QrkJ
This description is very hard to understand and the second sentence is a bit in dismay. Why not something like:
```
An enumeration type that denotes what kind of triangulation object (hex, quad, line, vertex)
a given degree of freedom is logically associated with. For continuous [...] this coincides with
the lowest dimensional object the support point of the degree of freedom lies on.
But for [...]
```