Add code for 2 remaining stratigraphic RESQML classes

The new strata.py module contains code for most of the stratigraphy related RESQML classes. This issue is to add code for the remaining 2 stratigraphy related classes:

- obj_GlobalChronostratigraphicColumn
- obj_StratigraphicOccurrenceInterpretation

Start by reading the new strata.py module and the existing stratigraphy-related RESQML classes to understand their structure and conventions. Implement obj_GlobalChronostratigraphicColumn and obj_StratigraphicOccurrenceInterpretation in that module, then verify that both classes are represented consistently with the other strata classes.

Written by the indexing model from the issue text.