MeshLib add properties

This PR is based on the discussion in PR #531:
- It adds an implemention to manage properties within the Mesh.
- The properties are associated to specific mesh items like mesh nodes or mesh
  elements via an enum class (see enum class MeshItemType).
- The properties are not restricted to a particular data type, you can use several types like double, vector or matrix
  types at run time together (see tests).
- There is not a possibility to get the value data type.
Put out an error msg that no prop of this name exists?