Support constraints of the format "x-y < c" in reachability query

[Mati-AAU]

Currently the reachability query only supports clock constraints such as "x < 2", "z >= 1", etc.

However those of format "x-y < c" should also be supported, by supporting the "-" operator.

This is currently missing from both the query parser and invariant parser (#96).

The reachability query parser uses the invariant parser, so that must be solved first.
After solving the invariant parser issue, making it work for reachability should be trivial.