Allow leaf on/leaf off to be specified as actual events

[Alomir]

As mentioned in #169, while we now report leaf on/off in events.out when we calculate it happens (due to param settings), it might be useful to be able to specify them as actual events (that is, create new events for them).

Suggestion:

• event type: phenology, or some shortened form of that
• params: type (on/off), date, relevant fractions/amounts (maybe have two events if they are too disparate)

For simplicity, I'll go further and propose:

• if there are any events of this type, the params that currently control leaf on/off are ignored. We only use one control mechanism, snd do not attempt to manage the two simultaneously.

[Alomir]

Note: consider this a 'draft' issue; we have not decided to actually do this yet.