rr_graph import: clean up pin import

Original idea was to clear the rr_graph nodes entirely and just re-add the pin nodes since its relatively straightforward to add them given the blocks. However, these nodes have placement on them (ie pin goes on left or right of block), which must match the arch.xml expectation after re-creating. I added some hacks on to export and then re-import these (see pin_sides() + add_nodes_for_pin_class()) but it might make more sense to just not clear the pin nodes. This should be fairly straightforward since they seem to be at the start of an rr_graph file (and thus we don't have to renumber them)

Some suggestions to resolution:
- If we want to preserve the existing nodes, run start suggestion by VPR devs
- Discuss / learn why pin placement should is part of the block vs in the node. It seems like usually it would not vary placement to placement. If this is moved in VPR, this issue goes away entirely