Include total runtime in Result string representation

[goerz]

Suggestion by @nathanshammah:

the print message during the optimization, instead of having start and end date and time, could just report the total running time.

I find absolute time stamps important, but having the duration reported in addition sounds like a good idea. Example output:

Krotov Optimization Result
--------------------------
- Started at 2019-04-11 20:07:54
- Number of objectives: 1
- Number of iterations: 679
- Reason for termination: Reached convergence: J_T < 0.001
- Ended at 2019-04-12 21:45:38 (1 day, 1:37:44)

where (1 day, 1:37:44) is the total running time (delta of "end" timestamp and "start" timestamp).