Enhance linter

Enhance linter to issue warnings when inappropriate fields are defined (for example, cores should probably not be defined on a user, destinations should be defining max_accepted_cores, not cores)

Also add warnings on unrecognised constructs, such as `elif`.

xref: https://github.com/galaxyproject/total-perspective-vortex/pull/58

Start by reviewing the linter changes discussed in pull request #58 and tracing the existing linter entry point and tests. Done means inappropriate fields produce warnings and unsupported constructs such as `elif` are reported, with coverage for both warning categories.

Written by the indexing model from the issue text.