Optional flag for tie-breaking vs tie-making

[kris-brown]

If multiple events occur at the same time (due to dirac delta functions), we have a choice between:

1. executing the events in an arbitrary order (some events may invalidate other ones scheduled for the same instant)
2. enforcing that the events must be compatible (other wise error) and then execute concurrently (Parallel independence AlgebraicRewriting.jl#60)

This could be a parameter to ABM. Finer granularity would involve opting in for specific rules to allow them to have automatic tie-breaking.

[kris-brown]

Parallel events get executed in an arbitrary order. It seems challenging (but possible) to check whether the order does not matter when there are PAC/NAC involved.