VPR: Add possibility to run extra python scripts in between VPR stages

[acomodi]

At the moment there is a very basic support for VPR which allows Edalize to create a very simple makefile to run the three main VPR stages, pack, place and route.

In F4PGA though we need to run a few scripts (e.g. place constraints, IO placement constraints) in between packing and placement to have the ability of constraining blocks to specific grid locations.

The VPR tool needs enhancements to allow running these extra scripts arbitrarily between the various stages, defined in the corresponding flow.

Practically this could mean adding either a new generic tool that can be used to invoke scripts with the corresponding options and enhance the VPR flow to call separately the various VPR stages, with the scripts calls in between when necessary, or enhancing the VPR tool/flow only without creating additional tools.