Elapsed Time for start and finish nodes

[fong22e]

• Start and finish nodes display elapsed time incorrectly.
• The time for those nodes is calculated in addStartFinishCollapsedNodes in PrefuseGraphBuilder by lastNodeTime - firstNodeTime in order to store the total elapsed time for the whole block.
• This is because the elapsedTime for each proc node used to be the time it took to finish that proc node since the start of the script's execution.
• Now it should be the sum of all elapsed times in the block.
• This can be done in RDataTracker itself, or in ProvParseR (creating a function to return the elapsed time for a node. If it is a start/finish node, calculate the time and return it)