Improved conservation of energy in case of feedback.

GEAR contains now an implementation of the sychronisation
of the particles timesteps and particule update
as proposed by Durier and Dalla Vecchia 2012.
This is enabled using the following flags:

-DSYNCHRONIZE_NGB_TIMESTEP
ngb particles have synchronized time steps

-DTIMESTEP_UPDATE_FOR_FEEDBACK
timestep is updated when feedback occurs

-DIMPROVED_TIMESTEP_CRITERION_FORGAS
improvement of timestep criterion for the gas