Fixed minor issue with NPT conserved energy

The value of the number of degrees of freedom in a group is a real,
but it was being truncated to an integer before promoting it back to a
real. This is wrong, whether or not the number can reasonably be
non-integral. This only affected the conserved energy quantity,
not the kinetic or total energy used for coupling and output.

Change-Id: I5fcb6428dc0e333cabd79262cc2a9cffa5fec03a