added Platin flowfact-to-PML export

Implemented as frontend action controlled by the -ff-export=<file> option.
Flowfacts (and other future platin annotations) can be placed in a pragma, only
simple, normalized flowfacts are supported for now:
  platin("-2 @1 + 3 @2 <= 4")
with @1, @2 referring to pc-markers.