schema: Add docstrings for intended behaviour ...

... not yet implemented.
schema: new field definition features, fixes + tests

as documented in the docstrings of the last commit:

- schema class definition: support "only" argument in __lima_args__
- schema instantiation: support "include"

also, make sure field overriding works as documented (with class var
fields clashing with __lima_args__['include'], but none of those
clashing with fields from base classes
Merge branch 'feature/fields-include-only' into develop
Update CHANGELOG.