Add 'Discrimation rule' property to harness branch class, allowing to disallow routed elements being routed through it.

- A new 'Discrimination rule' property will be added to the 'Harness branch' class.

- It will be possible to define a query in that property, which will be executed on the routed element (single wire, cable wire, etc.) for which SkyCAD is attempting to route through a harness.

- If the query returns 'TRUE', the routed element will be disallowed in that harness branch.

- Any other result will be considered as a 'FALSE' value, thus allowing the element to be routed through that harness branch.