How particle dt is determined

There are several ways that the inputs are used to determine what time step 

is used in the evolution of the fluid-particle system in MFiX-Exa.   

1) In a pure particle case, the "mfix.fixed_dt", if specified, is only used to determine the frequency

of outputs, it has no effect on the dt used in the particle evaluation. If you do not specify a positive

value of "mfix.fixed_dt" then the code will abort.

1) In a pure particle case, the :cpp:`mfix.fixed_dt`, if specified, is only used to determine the frequency

of outputs, it has no effect on the "subdt" used in the particle evaluation. If you do not specify a positive

value of :cpp:`mfix.fixed_dt` then the code will abort.

.. highlight:: c++

    amrex::Abort::0::If running particle-only must specify fixed_dt in the inputs file !!!

That is determined by the following calculation:

The particle time step "subdt" is determined by computing the collision time "tcoll" from particle properties,

then setting "dtsolid" to be "tcoll / 50".

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

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.

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

   the particle time step "subdt" is first computed as in the particle-only case ("dtsolid"), 

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