Consider using super idiom for child classes #124

Following discussion in pull request #110 (in particular, #110 (comment)), this issue is to decide whether to switch to using the super function when accessing members of parent classes.

Relevant reading in the Python documentation is https://docs.python.org/3/library/functions.html#super

At the current tip of master branch, I found three relevant parts of the code:

• line 208 of xferfcn.py
• line 153 of frdata.py
• line 128 of statesp.py