New core module *dune-geometry*

Today we introduced a new core module dune-geometry. It is intended for everything related to the computational domain that doesn’t really fit into a more specific module like dune-grid or dune-localfunctions. It initially starts out with Dune::GeometryType (from dune-common) and the generic reference elements, the quadrature rules and part of the generic geometries (from dune-grid). Kudos to Christoph GrĂ¼ninger for doing much of the work.