add support for the UG grid library

recently UG has become free software (yay!), so we can now support it
with a good conscience. The CMake module is based on the "UseUG.cmake"
module of dune-grid but it is quite a bit simpler and actually
works. (I cannot see how UseUG.cmake can work without ever including
the ug-config*.cmake files...)

the UG source code is available here:

https://github.com/ugfem/ug