change makefile to use git instead of svn

We called svn status during make clean, which should be git status now
add function to compare mpi/farc ddt extent and lb
compare farc/mpi ddt extent and lb bound in tests
change the way we calculate a ddts extent

Before we set/caclculated the extent of a datatype directly. This is
incompatible with MPI as MPI defines the extent to be ub-lb, and offers
functions for querying those. This commit provides such functionality
for libpack.