Add convenience typedef.

I'd like to provide helper typedefs for structs like GradientBasis that provide a type in order to access it conveniently. In the standard library those are given with suffix _t, leading to GradientBasis_t. However, this is in conflict with Dune's CamelCase policy. A convenient alternative IMHO would be to use GradientBasisType. Unfortunately, this kind of construct conflicts heavily with how template parameter typedefs are handled in many classes. My feeling is that in this case, a deviation from the CamelCase rule is acceptable.

I did not find an existing case to whose style I could adhere to, although I have to admit that searching for this kind of thing is quite difficult.

Edit: Was merged despite CI build not being successful. The changes are non-invasive though and the build problems also occur in the current master branch, supposedly due to upstream changes.