Beta property reference to `3DModel` has broken the generated libraries

[nickevansuk]

The introduction of a beta property (openactive/modelling-opportunity-data#306) that referenced 3DModel broke some of the model libraries generated by this generator.

A temporary fix has been introduced by:

• Temporarily removing this from beta openactive/ns-beta@0ce65b1
• Adding an exclusion to the validator tests https://github.com/openactive/data-model-validator/blob/5c46a00971dfe16e7221680b89ff1a6a287f0290/src/examples-spec.js#L32-L34

This needs proper debugging to figure out the issue in each library.

Possibly related to #56