Make omega_mass proportional to (N+1)kT

According to papers like Martyna, Tobias, Klein (JCP 1994,
doi:10.1063/1.467468 section II.F) and Martyna, Tuckerman, Tobias,
Klein (Mol. Phys. 1996, doi:10.1080/00268979600100761 section 2.5), the
mass of the cell parameters should be proportional to (Ndof + dim) / dim, or
in other words, Natoms + 1.