Proposed changes

This adds support for parsing PDDL files with :axiom statements (defined here).

Fixes

Fixes #65

Types of changes

• adds :axiom, :vars, :context, :implies statements to the lark language specification
• adds parsers for them in the DomainParser
• adds an Axiom class to pddl.logic.axioms

Checklist

• I have read the CONTRIBUTING doc
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

Further Comments

I defined axioms using the same syntax symbols as actions (context -> precondition, effect -> implies), if you could confirm that this is correct, or if I'm missing some nuance that would be great, thanks!

?axiom_body_def: [CONTEXT emptyor_pregd] [IMPLIES emptyor_effect]