Fix grompp with Andersen massive and no COM removal

Fixed a floating point exception leading to a segv.
Also fixed possible different rounding for the interval for
Andersen massive in grompp in mdrun for the common case where tau_t
is a multiple of delta_t.

Fixes #2256

Change-Id: I161e8a9db2c31fde8a6e8c2fd32551b21423fd9b