This PR puts in some unit tests associated with issue #935, which looks like it is a problem in slycot where the tf2ss functionality (in td04ad) doesn't work properly. As a partial workaround, a method keyword is added to tf2ss (and ss) that allows method='scipy' to be used for SISO systems. Docstrings updated for the new keyword and a note on the slycot issue.

A bug report has been created in the slycot repository (#222 and the unit tests will fail when the bug is fixed (so we can go back and update the unit tests and documentation).