Simplify the collection of daily HSI events for mode 1

[tamuri]

• Collecting index of alive individuals repeatedly allocates memory and becomes slower than querying directly (esp. on large populations)
• In mode 1, HSI events have the same priority so queue is effectively ordered by date. Stop processing when we reach the events from the next day
• If health system is running in mode 1, then always ignore priority setting (all events get priority 0)
• If we switch from mode 1 to mode 2, update every HSI event in the queue with enforced priority

[marghe-molaro]

This looks good to me!