New functions augw and mixsyn modelled after Matlab and Octave's
function of the same names.

python-control doesn't currently support input group naming, so that
is not implemented.

Both examples taken from Skogestad and Postlethwaite's Multivariable
Feedback Control.

control/robust.py:
  - doc-strings conform to python-control conventions
  - use plain tuple instead of namedtuple for auxiliary output in mixsyn

control/statesp.py:
  - don't import pi from numpy
  - doc-string for _matrix expanded, conforms to conventions

control/tests/robust_test.py
  - change test to accept plain tuple