Make explicit (optional) validation function in TrackingGraph constructor

[cmalinmayor]

This will be more efficient for power users who know the networkx graph is correct, and for creating subgraphs of the TrackingGraph. Should probably put the validation in a separate function and call it or not based on the argument.

[cmalinmayor]

I just ran into the issue that the label_key was incorrect. We should check that if there is a segmentation provided, every node has the label_key in the validation as well to avoid this issue (or throw a warning if we don't want to be strict). The actual error message I got was a KeyError in the CTCMatcher which was very uninformative.