Validate models against XSD schema

We could validate the XML part of models against the published RESQML schema, to check for errors.

This could be available as a function `validate_model` or similar. It could also be used in our test suite, to ensure we are creating each object correctly.

RE implementation, lxml looks promising as it is already a dependency:

- Example here: https://lxml.de/validation.html
- This class looks like it can be used for validation: https://lxml.de/apidoc/lxml.etree.html#lxml.etree.XMLSchema

We would presumably need to have the XSD files available to the code. We should probably check whether we're allowed to include them in the repo; and in the mean time, we could try to get it working against a local copy of the XSDs outside version control.

Start by reviewing the proposed validate_model entry point and the lxml XMLSchema documentation, then determine how a local copy of the RESQML XSD files can be supplied. Check whether the schemas may be included in the repository and consider how validation would be used in the test suite; done means model XML is checked against the published schema and errors are reported.

Written by the indexing model from the issue text.