update test_armax.ipynb

[imgeorgiev]

Motivation

The test_armax.ipynb was outdated in its usage of PRBS_seq which is no longer part of scipy and thus breaks the notebook. Additionally, some of the plots were difficult to read due to the lack of legends.

Changes

• Modified all usages of PRBS_seq to the newer and working GBN_seq scipy function.
• Added legends to some plots and refactored the legends of other plots to be more robust to changes.

Testing

Tested and worked with python 3.7.5 and scipy 1.3.2

PS. The other examples don't seem to have this issue.

[CPCLAB-UNIPI]

Thanks for updating this notebook for the ARMAX example. Note that GBN_seq function (previously named PRBS_seq) is internal to SIPPY and not scipy.