Fix the target kinetic energy of the NH barostat

The cell momenta should be thermostatted to kT per barostat degree
of freedom (d^2 in general, d*(d-1) without rotations), according to
Shinoda et al. 2004 (doi:10.1103/PhysRevB.69.134103) Eqn. 1 and Martyna,
Tobias, Klein (JCP 1994, doi:10.1063/1.467468 section II.D).