Incomplete checks for supported data location types

[MakisH]

#206 adds some sanity checks on whether the user is asking for a valid combination of data type (displacement, force, ...) and location (face centers / face nodes).

However, this does not currently cover cases in which we can only read but not write (and the reverse), asumming that the user knows what she is doing. This is a bit dangerous, because it creates a false impression of enough checks for every case. However, it was necessary because the rest of the code also does not nicely handle this distinction between reading and writing.

We could extend/redesign the code to make such checks easier.

Motivated by #206 (comment).