Update rpmdrate/_math.f90

pls add "integer i "
Use geometry from umbrella configurations in recrossing factor calculation.

Previously we were always using the xi = 1 transition state geometry.
This was probably fine for xi_current near one (since constrained
equilibration would move us to the correct actual geometry), but doesn't
work for xi_current far from one. The problem manifested as a SHAKE/RATTLE
constraint failure, which is not surprising since the input geometry
could have been very far from the constraint!