KSP solver divergence in KHI case

[shyams2]

Commit Hash: 1bb6be9

Issue:

• Error of 'KSP solver diverging!' is thrown when main.py from $BOLT_HOME/example_problems/nonrelativistic_boltzmann/instabilities/KH/hydrodynamic is run.
• The KSP solver seems to have worked fine for all linear modes I.C setup
• The KSP solver passes its unit test. Need to add case which replicates the scenario occuring in this case.

[mchandra]

The Poisson solver only sees the density:

    q2_minus = 0.5
    q2_plus  = 1.5

    regulator = 20  # larger value makes the transition sharper

    rho = 1 + 0.5 * (  af.tanh(( q2 - q2_minus)*regulator) 
                     - af.tanh(( q2 - q2_plus )*regulator)
                    )

1. Can you make a unit test with the above density profile and use the FFT solver as the reference solution?
2. In that unit test, try changing the regulator and check if the KSP solver works below a certain value.
3. Paste a plot here of the above density profile.

[shyams2]

@mchandra Needs to be rechecked with the change to SNES