FLASH4 complete API

 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.