Proactive IV switching on weather change

[Mygod]

Currently we keep track of both sets of IVs and in principle we can proactively switch IV. Basically it will be like this WatWowMap/Chuck#59, but we can probably skip updating any record without an active IV set.

It will be useful in the following scenarios:

1. Weather changes twice (nonboosted-boosted-nonboosted). This is quite rare but could happen when a/b weather/spawn occurs.
2. Something like that but involves spawn reroll instead. This could happen if there's a 5-minute mini event.
3. A non-pc-boosted ditto becomes pc-boosted. For example, a sunny Litleo-disguised Ditto can become partly cloudy and still boosted.

Before implementing this though, @jfberry suggests that

if we can derive extra information in a sensible way and we balance the complexity and performance considerations I'm all for it.
I am suggesting a cautious approach to documenting the real situation and working through some examples - given what we know about when encounters and gmos arrive and how they are sequenced