[fvvelocity1p] store geometry reference

Without storing the geometry, the call
  const JacobianTransposed& jacT = eIt->geometry().jacobianTransposed()
effectively resulted in an all-zero matrix, once optimization >=-O1 is
turned on.

This is very disturbing, since such code might be in quite some places.
We have to investigate this thoroughly.

Discussed with and reviewed by Martin S.



git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@13509 2fb0f335-1f38-0410-981e-8018bf24f1b0