Should we have a Sample3Phase at all?

[llucax]

When #880 was closed, I started to think why do we have a Sample3Phase at all. This is just grouping 3 different metrics.

Specially with the new microgrid API, for which we even have a separate name for each phase metric, it seems like it doesn't make a lot of sense to have this special grouping.

Many times we need to group metrics together to make sure all metrics belong to the same timestamp, so we should have a generic solution for this. Even more, we will start to get phase-to-phase values too, which would be better to fit in this model (as they are a 6-values tuple).

Having a special Sample3Phase makes things more complicated too, we need to handle one more case on all the formula engine stuff, which is quite different to other cases (float/quantity which are single value). In terms of math you need to know how to deal with tuples, etc.

My suggestion is to get rid of Sample3Phase and instead have some sort of SampleSynchronizer that takes several receivers and gives you a new receiver that outputs something like:

class SampleGroup:
    timestamp: datetime
    values: tuple[T, ...]

If we go this route, probably it makes sense to do it as part of the migration to the new microgrid API version.