[draft] Add support for raw pointers as CornerStorage

By using Dune::resolvePtrOrRef() we can support raw pointers as CornerStorage in MultiLinearGeometry and thus have a non-owning corner storage while still being default constructible.

The test adds a third test case with raw pointer as CornerStorage and also checks default construction and assignment of MultiLinearGeometry if the CornerStorage is default constructible.

Warning: This is a DRAFT implementation to demonstrate the approach.

This is only meant to demonstrate what was proposed in !224 (comment 130753).