Remaining issues for vocs

[shuds13]

Points deferred from #281

Points that could be addressed in follow up PRs (tick means discussed and added to new issue).

• Update validate_vocs functions and incorporate previous check functions.
• Add tests for validate_vocs exceptions
• Decide whether to remove extra fields given to gens (_ignored, _ignored_reason, _status).
• Verify that the fields passed between ingest/suggest methods are correctly derived from VOCS specifications (due to keeping the old internals, the generators work with Optimas legacy parameter handling). Indirectly this happens.
• Add xopt generator example.

Priority - support needed for xopt generators - see #293

• Constraints are given in tests, but are currently ignored in Optimas. How to deal with this.
• Constants are not used currently in Optimas generators, we could add tests or leave until needed.

Checks:

• Check/update docstrings and other references to old data structures
• Check and update validate_vocs functions
• Check notebooks in docs
• Check docs