WIP: Updates to steady state module

Added ARPACK eigensolver, updated power method to use RCM, start from
maximally mixed state, and not recalculate LU factors everytime solver
is called.
This function duplicates `_version2int` from the init file, which should probably be replaced with this function. Using tuples instead of int is nice, but `_version2int` handles some special version string formats like `0.20.1post` (which this function fails to parse). We should take the list of special cases processing from _version2int and add to this functions, like this:

    v  = v.split("-dev")[0].split("rc")[0].split("b")[0].split("post")[0]
    return tuple(map(int, (v.split("."))[:3]))