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.
@rinkk I think this method is creating a new PropertyVector; maybe more explicit, like `createNewProperty`.
@TomFischer What happens, if a PropertyVector with the same name and the same type already exists? => documentation