This routine implements the time advancement scheme being used in a simulation. The default 'Unsplit' version implements a directionally unsplit integrator for time advancement. A single step in this driver accounts for all multidimensional fluxes and updates conserved variables for the next time step. The routine is a default driver for the Unsplit Staggered Mesh (USM) MHD and Unsplit Hydro solvers. An alternative version provided with FLASH is appropriate for a Strang splitting scheme; this used to be the default implementation before FLASH 4.4. A single step in this driver includes two half-steps, each advancing the simulation by a time 'dt'. Of these two half-steps, the first one calls a Hydro sweep in order XYZ, and the second one in order ZYX. This driver should be used with directionally split operators (in particular for Hydro) only.