densitymx_slow does not work

[aristaeus]

Describe the bug The densitymx_slow implementation does not work. At pygsti/evotypes/densitymx_slow/opreps.py:177,

class OpRepStandard(OpRepDenseSuperop):
    def __init__(self, name, basis, state_space):
        ...
        super(OpRepStandard, self).__init__(superop, state_space)

The issue is that OpRepDenseSuperof.__init__ doesn't have that signature;

class OpRepDenseSuperop(OpRep):
    def __init__(self, mx, basis, state_space):
        ...

To Reproduce Steps to reproduce the behavior:

1. Install pygsti without cython
2. Try and run a circuit
3. :(

Expected behavior Ideally we would get the fast cython implementation working. However, while setting up a new user it would be nice for the slow fallback to work so we can get something out.