Run replica-exchange tests with MPI enabled
Replica exchange only works with real MPI and at least two ranks. This
can now be handled by ctest, so we can run such tests automatically.
The other integration tests run in MPI mode, but do not place any
requirements upon the number of ranks, so they continue to run in the
usual way.
Fixed the replica-exchange integration tests so that they actually do
run. Changed tests to use mdrun -multi because it is easier to write
the tests that way. (Using mdrun -multidir is problematic, because
grompp needs to write the .tpr file in the subdirectories. When using
mdrun -multi, the only issue is that the name of the .tpr file for
grompp and mdrun is different, because the latter will append the MPI
rank.)
Fixed member variables to conform to naming convention.
Refs #1587
Change-Id: I11dc06f3aac81a80d679b036aef24762e9eec819