Removed truncation of nrdf in v-rescale thermostat

The resampling function for the v-rescale thermostat expected an
integer value for nrdf, but a real was passed, which was truncated.
With a single coupling coupling group nrdf is analytically an int,
but could be off by a bit. The could lead to incorrect kinetic
energy fluctuations (averages were correct).
Now fractional nrdf's are properly handled for nrdf > 3.
For nrdf < 3 a check is added for integer values with a small margin
for rounding.
Fixes #1218

Change-Id: I4c60c337f9874d0bff51220ad09429140be2a056