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

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.

Start by reading the invariant parser and the details of issue #96, since the reachability query parser uses it. Add support for the '-' operator in invariant constraints of the form "x-y < c", then verify that the reachability query parser accepts the same format. Done means both parser paths support this constraint.

Written by the indexing model from the issue text.