Define thresholds for HLT1 MC production

For #192, we will need all HLT1 lines to have a set of thresholds which reflects the HLT1 output rate constraints and our physics goals in HLT2.

We should consider having multiple sets of thresholds for one or more lines, as we had in the HLT1 filtering productions for the HLT2 bandwidth studies. These were useful as HLT2 line authors were able to characterise their HLT2 lines under several different HLT1 output rate scenarios.