Add `operator<` for Trigger(Primitive|Activity|Candidate)

[philiprodrigues]

A handful of places in trigger rely on sorting Trigger(Primitive|Activity|Candidate) by start time. It might be better to "bless" that ordering by adding an operator< for each of the classes

[ArturSztuc]

This is already done in TriggerPrimitiveAdapter, TAWrapper and TCWrapper, but could be done in the definition of Trigger(Primitive|Activity|Candidate) too.

I assigned this issue to v5.x.x target release: will discuss at the Data Selection meeting (and/or Core SW), and see whether to close this issue (since done in the wrappers) or have these in the primary objects themselves.