Revert deprecation of GeometryType::BasicType for 2.6
As there are still some use cases that cannot be easily switched from using GeometryType::Basictype to plain GeometryType, revert its deprecation for the 2.6 release.
This does not revert the deprecation of the GeometryType constructor from BasicType.
See #17 (closed), pdelab/dune-pdelab#102 for further explanations.
Edited by Steffen Müthing