TEST: Add tests for arma.py

[mmcky]

Coverage:

arma.py                                123     83    33%

needs additional tests. There are a number of plots that will affect coverage.
Should we test simple executability of these methods -- or add image comparison tests (http://matplotlib.org/devel/testing.html#writing-an-image-comparison-test). I tend to think exectuion is all we are after here and image testing will probably be a headache.

[jstac]

Plot functions removed, closing

[oyamad]

It seems some tests are still not covered.