Stable 4.3.3

This version of the code includes a large change brought from 4.2 -> 4.3, which is support for multiple assignments. Now the LineList object is able to store references to multiple transitions that were possible during the assignment procedure, and will allow the user to choose from them.

Other changes include minor bug fixes, and one in particular brings it up to date with the latest pandas>=0.21.0 release.