ECOO cumtime improvements

[kiritofeng]

cumtime should be extended to allow for 4 possibilities:

1. No cumtime
2. Current implementation (i.e. every submission increases cumtime, sum over all problems), called all
3. Sum of times based on last submission, but only if there was a non-zero submission to the problem at some point, called sum
4. Maximum cumtime over all problems that had non-zero score at some point in time, called max