Calculating forces in reftraj runs

[Ondrej Marsalek]

Dear CP2K developers,

I am doing reftraj runs where I am interested in some electronic
properties, but not in the forces on atoms. It seem that these are
always calculated, though, with no setting in the input file - the one
that is available is energy+forces or nothing. I have tried disabling
forces in `force_env_methods.F` (just changing `calculate_forces`)
and, as an example, with a hybrid functional I got times per step go
down from ~135 s to ~115 s. While this is not a deal breaker by any
means, it would be nice to have a settings for reftraj, where you can
enable the calculation of the forces separately. My guess would be
that on line 1674 of `motion/integrator.F` the call to
`force_env_calc_energy_force` should get `calc_force` passed to it,
which would be read from a new setting in the REFTRAJ section.

Thank you for considering this,
Ondrej