Add ProtocolDAGResult.total_time() convenience method for getting total execution time

[dotsdl]

Currently, to get the total accumulated execution time of a ProtocolDAGResult, one must extract the start and end times of each ProtocolUnitResult and sum them up, like so:

pdr: ProtocolDAGResult
pur: ProtocolUnitResult

time = sum([(pur.end_time - pur.start_time).total_seconds() for pur in pdr.protocol_unit_results])

It would be valuable to have a convenience method, such as ProtocolDAGResult.total_time(), that performs this summation and returns the total time in seconds.

This method could also optionally take a keyword-argument to indicate whether or not to include ProtocolUnitFailures in its summation. If not, it would sum over ProtocolDAGResult.protocol_unit_successesinstead of allprotocol_unit_results`.