Partial start state for reachability query

The reachability query currently supports partial end states, but not partial start states.

Partial states are states where atleast one location is marked as being any location.
This means that a partial location represents many states in a transition system.

This will require changes in the grammar, the parser, and the algorithm itself.

Will probably work similarly to partial end states in the grammar and parser, however will be implemented differently to partial end states in the algorithm itself.

I think the easiest way to implement it, is to in the find_path or reachability_search_algorithm function, to iterate through all states that the start state represents, and add each of them to the frontier, instead of only adding one state to the frontier, which should be correct and also be performant.

Examples of queries with partial start states where it should return reachable.
Machine -> [_](y>6); [_](y<2)
Machine || Researcher -> [_, _](x>6); [_, _](x<2)
Machine || Researcher -> [_, _](y>6); [L5, _](y<2)

Start by reading the grammar and parser handling for partial end states, then inspect find_path or reachability_search_algorithm to understand how the start state enters the frontier. Add partial start-state support for the examples in the issue, and verify that each example returns reachable while existing partial end-state behavior remains correct.