…ects.Allows StateSpace([],[],[],D), which failed previously.  Static gainshave sizes enforced as follows: A 0-by-0, B 0-by-ninputs, Cnoutputs-by-0.Tests added for instantiation, and sum, product, feedback, and appending,of 1x1, 2x3, and 3x2 static gains StateSpace objects.

On Python 2.7, the special case "all states useless" in_remove_useless_states resulted in A=[[0]] (and similarly for B and C).The special case is no longer needed, since empty A, B, C matrices canbe handled.  numpy.delete does the right thing w.r.t. matrixsizes (e.g., deleting all columns of a nxm matrix gives an nx0 matrix).Added test for this.

Do this by only calling Slycot's tb01pd for non-static systems.