Migrate ome_model Python package to its own repository

[sbesson]

The code under https://github.com/ome/ome-model/tree/v6.3.6/ome_model has had little maintenance/development over the last 4 years.
Functionally, its content is also very different from the rest of this code base which contains:

1- the source XSD files for the OME data model
2- sample XML files for the different schema versions
3- the code-generated Java implementation of the OME model objects

Following the extraction to a separate repository, a decision should likely be made with regard to maintaining it vs archiving it in favor of other community Python projects like https://pypi.org/project/ome-types/.

Originally posted by @sbesson in #193 (review)