BUG: No dt in StateSpace operations

[AndyLamperski]

When adding or multiplying discrete-time StateSpace systems, the "dt" gets dropped, so that they become continuous time systems.

Reproducing code example:

M = np.ones((1,1))

P1 = sig.StateSpace(M,M,M,M,dt=1)
P2 = sig.StateSpace(M,M,M,M,dt=1)
P3 = P1 + P2
print(P1.dt,P2.dt,P3.dt)

This gives an output of

1 1 None

I think that all that is needed is to fix add the "dt" to the return statements of add, mul, and so on:

        return StateSpace(np.asarray(a, dtype=common_dtype),
                          np.asarray(b, dtype=common_dtype),
                          np.asarray(c, dtype=common_dtype),
                          np.asarray(d, dtype=common_dtype),
                          dt=self.dt)

However, I can't get the github scipy to compile correctly (due to #8325), so I have not tested this.

Scipy/Numpy/Python version information:

import sys, scipy, numpy; print(scipy.__version__, numpy.__version__, sys.version_info)

1.2.1 1.14.3 sys.version_info(major=3, minor=6, micro=8, releaselevel='final', serial=0)