This solver will implement a basic low Mach number solver for atmospheric flows.  
We need to start with the incompressible solver and:

 -- add density evolution

    o switch constraint to div (beta/rho) grad phi = RHS

    o advect rho

 -- add support for boundary conditions other than periodic

 -- add a base state functionality

    o define this from the initial state by taking rho_0 = < rho > and
      then p_0 by differencing HSE.  We will do FIXED in time.



Problems:

 -- bubble rise (which can be compared to the compressible solver)


Known issues:

-- if proj_type == 1, we need to use the correct coefficient for gradp

-- there seems to be a break in the symmetry

-- at the moment, this does not work with runtime visualization -- no idea why,
   but rho goes NaN if we run with runtime visualization.
   