RatesAnalysis: Fixes related to rate calculation of disabled items

• fix calculating the rate for disabled triggers (function was only treating the prescale = 0 for a disabled trigger, where it should be <= 0)
• do not print express stream prescale/rate for L1 items
• fix the "Multiple" prescale category (disabled chains with prescale = -1 were treated as ones with multiple seeds)
• do not calculate the unique stream rate for disabled triggers

Example (details in jira, 100 events) https://atlas-trig-cost.web.cern.ch/?dir=aporeba&type=&tag=negative-rate-7&run=00440499&range=All

Reference without the changes https://atlas-trig-cost.web.cern.ch/?dir=aporeba&type=&tag=negative-rate-8&run=00440499&range=All

Comparison https://atlas-trig-cost.web.cern.ch/?page=CompareRates&key=64b6ba7142935

Tagging @tamartin @khoo

Related to ATR-27867