Update info on time stepping

2) In a pure fluid case, there are two options:

  * If you want to fix the dt, simply set :cpp:`mfix.fixed_dt = XXX` and the fluid time

    step will always be that number.  [SHOULD WE ABORT IF YOU ALSO SET CFL?  OR SPECIFY

    THAT FIXED_DT OVERRIDES CFL?]

    step will always be that number. 

  * If you want to let the code determine the appropriate time step using the advective CFL

    condition, then set :cpp:`mfix.cfl = 0.5` for example, and the fluid time step will

    condition, then set :cpp:`mfix.cfl = 0.7` for example, and the fluid time step will

    be computed to be dt = 0.5 * dx / max(vel).

Note that these choices apply to steady state calculations as well as unsteady runs.

      * If dt as computed in the compute_dt routine is smaller than the user-specified 

        ```mfix.dt_min``` then the code will abort:

.. highlight:: c++

::

    amrex::Abort::0::"Current dt is smaller than dt_min !!!

      * If dt as computed in the compute_dt routine is larger than the user-specified 

        ```mfix.dt_max``` then dt will be set to the minimum of its computed value and dt_max

      * If dt as computed in the compute_dt routine is larger than the user-specified 

      * Note that the cfl defaults to 0.5 so it does not have to be set in the inputs file. If neither

        :cpp:`mfix.cfl` nor :cpp:`fixed_dt` is set, then the default will be the second option here.  

        If :cpp:`mfix.fixed_dt` is set, then it will override the cfl option whether 

        :cpp:`mfix.cfl` is set or not in the inputs file.

These options apply to steady state calculations as well as unsteady runs.  

3) In a coupled particle-fluid case, dt is set as in the pure-fluid case.  In this case

   the particle time step is first computed as in the particle-only case, then is adjusted

   so that an integral number of particle steps fit into a single fluid time step.

[HOW ARE DT_MIN AND DT_MAX USED???]