[ML] Data structure for storing properties.
[ML] Impl. of addProperties() and getProperties().
[ML] Docu in class mesh: addProperties() / getProperties().
[T] Test for properties in class Mesh.
[ML] Move property implementation to separate file.
[ML] Add MeshItemType parameter to getProperty and addProperty().
[ML] Impl. of PropertyVector.
[ML] Using PropertyVector in class Properties.
[T] Update test.