Novel PAM-C Time Stepping

[celdredsandia]

For both Anelastic and Fully Compressible
-Positivity-Preserving Energy-Conserving Semi-Implicit, if this is possible
-Multiderivative Runge-Kutta (fully compressible will need the IMEX/HEVI version of this)
-Relaxation Runge-Kutta (fully compressible will need the IMEX/HEVI version of this)
-Invariant-Domain Preserving Runge-Kutta (fully compressible will need the IMEX/HEVI version of this)
-Adaptive Time Stepping (fully compressible will need the IMEX/HEVI version of this), especially relaxation, SSP and invariant-domain preserving versions

For Fully Compressible
-HEVI- KG, SSP and "energy-conserving" variants

Basically, what we are searching for here is a scheme with large explicit time step (both the ability to handle sound waves in the fully compressible case and a large imaginary stability region) that preserves total energy and also positivity. Our existing schemes typically get just one or two of these properties:

-semi-implicit allows a long time step and preserves energy but not positivity
-KGRK (and IMEX/HEVI variants) allows a long time step but doesn't preserve energy or positivity
-SPP (and IMEX/HEVI variants) preserves positivity but doesn't allow a long time or preserve energy

Of the three properties, we can probably give up exact energy conservation the easiest