Don't use deprecated np.matrix in tests.

[roryyorke]

This silences deprecation warnings when running tests with newer (1.15?) Numpy. I've only tested it on my own machine (Ubuntu 18.04) with Python 3.6; I'll recheck with other versions when #38 is merged.

This PR also serves as a proof-of-concept of the sort of changes we'd need to make for python-control/python-control#233 (see commit message below for details). Luckily we don't seem to use the .H or .I attributes anywhere. I'm not so sure about .A, which is more difficult to grep for since SS objects have a .A attribute.

commit message:
This involves

1. changing np.matrix to np.array, or removing conversion from array
   to matrix, and in one case by-hand converting "string
   constructor".
2. replacing np.matrix "*" with np.array ".dot"; since we're still
   supporting Python 2.7, can't use "@".