Corrected read/write of velocity units between ASE and gpumd

This MR fixes the unit mismatch between GPUMD velocities and ASE velocities by including a conversion factor from ASE's units module into calorine's gpumd.io module.

GPUMD uses Å/fs while ASE uses Å/Åsqrt(amu/eV). Prior to this MR, setting a temperature with ASE's VelocityDistribution would initialize the GPUMD simulation as ~100x hotter than specified.