Fix athena CI

This fixes three problems in Athena CI runs:

1. Fix Athena CI for nEventsPerJob<100
   The minimum number of events to run in central production should not be 10 but 1.
2. Make negative XS check into warning instead of error
   Negative XS can appear for two reasons: statistics (only small number of events run in CI test jobs), and special samples (e.g. interference) where the cross section is correctly negative
3. Change optimal CPU time to 6-12 h
   The previous 8 - 12 h is not always possible to achieve since allowed nEventsPerJob are a factor of 2 apart.

Closes #62 (closed) #59 (closed)