fixed floating species defined by rate rules -- we only store amounts,
so need to convert concentration rate of change to amount rate of change.
Careful to take product rule into account in the case of volumes
defined by rate rules.

Fixed event priority comparator to compare on event assign time rather
than event delay time -- higest priority is least assign time.

Get event persistent type for l2v4 doc, for l2v4 docs, if we treat
all events as persistent, it seems to work.