properly implement __repr__

The `__repr__` methods should implement a representation that can be 'eval'-d back into the relevant object, so
```
>>> from control import TransferFunction
>>> s = TransferFunction.s
>>> h = 1/(s+1)
>>> print(repr(h))
TransferFunction([[array([1])]],[[array([1, 1])]],0)
>>> from numpy import array
>>> h2 = eval(repr(h))
>>> print(h2)

  1
-----
s + 1
```

