Kessler microphysics updated

[nataraj2]

This PR adds the Kessler microphysics which is a simpler version which has only rain - no snow/ice/graupel.

1. The background moist hydrostatic initial condition is determined using forward integration combined with a Newton-Raphson procedure as for the dry case.
2. The initial condition for potential temperature and qv matches the WRF test case as in figs. 1 and 2.
3. The hydrostatic condition with moisture is satisfied as shown in Fig.3 which shows the z-velocity after 100 s of runtime - the velocities are of order 1e-13 m/s showing that the hydrostatic balance is satisfied.
4. The squall line case can be run (see Fig. 4 showing contours of cloud water at t = 5000s)

The docs will be updated in another PR soon.