Proper use of NXtransformations and NXcoordinate_system(_set)

[lukaspie]

In order to present different coordinate systems and the transformations betwen them, it would be benificial to extend these classes such that:

1. in TRANSFORMATIONS depends_on, one can place either ".", or the path to an NX_coordinate_system
2. in NXcoordinate_system, there is a depends_on, and TRANSFORMATION field
3. if a depends_on attribute or field links to a NXcoordinate_system, it shoud pick the respective depends_on field in that class, and apply the specified TRANSFORMATIONS
4. NX_coordinate_system_set contains NXcoordinate_systems