[CMake] Add a helper function to extract paths

Developing CMake code yourself you often need to access
path variables like: What is the build directory of dune-grid?
What is the cmake modules directory of dune-common?
Unfortunately, there are two problems with that:
* The extract method changes depending on whether you are currently
  configuring the module itself or whether you are configuring a
  downstream module.
* Normal people tend to forget the naming schemes of the path variable.

This commit introduces a function with a documented interface that
allows for querying dune module paths. You give it the name of the
dune module, the name of the return variable and a path type.
Currently implemented path types are CMAKE_MODULES and BUILD_DIR.
More may be added as needed.

See merge request !80