Add snapshot triggers to start recording elements X time units before a snapshot dump

Features:

• Read the list of trigger times from the parameter files,
• Every step we check whether we passed a threshold and if we did, raise a flag,
• The EAGLE tracers aggregate the average SFR over these periods,
• The BH aggregate accretion rates.

Todo:

• Documentation
• Update parameter files for EAGLE-based examples
• Verify output lists to prevent overlapping recordings.