OpenMesh improvements: more mesapd functionality, improved cmake

This adds the "contains" functionality for mesa_pd's convex polyhedron particles. This functionality is required for coupled fluid-particle simulations to obtain the correct mapping into the fluid domain. It is equivalent to the functionality already provided by the pe before. Additionally, a test has been added.

Doing this, it has been noticed that the way of activating OpenMesh is misleading in that the default way was to specify OPENMESH_LIBRARY_DIR to the directory of OpenMesh (and NOT of OpenMesh/lib). However, this variable is then overwritten in FindOpenMesh.cmake to OpenMesh/lib. Thus, a new CMake variable called OPENMESH_DIR is introduced to clearly distinguish between the parent folder (i.e., the OpenMesh directory) and its library directory. In order to activate OpenMesh, one thus should use the CMake options as e.g. -DWALBERLA_BUILD_WITH_OPENMESH=ON -DOPENMESH_DIR=/software/openmesh/X.0 (the alternative is to preprend /software/openmesh/X.0 to the CMake prefix path as e.g. done by the module command. A corresponding message is now printed if OpenMesh is not found but is looked for.