Extract transition list from two Vega-Lite specs.

[yhoonkim]

Extracting a set of transitions enabling Vega-Lite spec u to become spec v.

Input : Transitions with costs (Tr), start Vega-Lite spec (u), destination Vega-Lite spec (v) Output : a part of Tr on the cheapest path (in terms of the sum of the costs on the path).