remove the compressible_sr solver (#226)

this algorithm is not really documented and we don't have
a lot of description in the pyro docs.  It is also not actively
developed.

we can always revive it from git if we decide to pursue it more

Author: zingale

docs/source/pyro.compressible_sr.problems.rst

@@ -24,7 +24,6 @@ Subpackages
    pyro.compressible_react
    pyro.compressible_rk
    pyro.compressible_sdc
-   pyro.compressible_sr
    pyro.diffusion
    pyro.incompressible
    pyro.incompressible_viscous