Implement the MPC use case for Monitored Units

[sthelen-enqs]

This adds an implementation of the MPC use case for Monitored Units.

The current implementation has 2 major restrictions so far:

1. it automatically configures itself as a 3-phase meter (ABC) and there's no way to specify a different configuration right now
2. it only supports the required phase-to-neutral and not the phase-to-phase voltages

I'm opening this as a Draft PR because while this UC works, it can still be improved and I'm looking for feedback and potential improvements. Some of the areas I'm explicitly looking for feedback in are:

• how best to make instantiation of the use case configurable (number of phases supported etc)
• how best to adapt the public usecase API when phases are configurable (e.g. adapting functions to take []float64 or something else entirely)
• how to improve the current tests, I thought about connecting a remoteDevice and reading the set values out via that but couldn't get that to work easily, the alternative as far as I can see would be to add functions to the public api and copy the cs/lpc tests

[DerAndereAndi]

Thanks a lot for the PR!

Some first thoughts, more will surely come:

Having individual public functions for each scenario is what surely is expected and needed. But using those individually will result in a notify update message for each call, which I think is not nice.

I do see 2 options here:

• Add another public method that can take all measurement values for all scenarios, with all being optional at the end so only the ones with actual data will be used
• Introducing a mechanism in the SPINE repository to combine multiple local data updates to work like a transaction, where only the transaction end will trigger the actual local data storage update which will then automatically trigger the notify messages to all subscribers

[DerAndereAndi]

Regarding initialization: how about the possibility to define the supported scenarios, and for each scenario define a configuration item where items like amount of phases, measurement type, reference to etc. needs to be set.

Scenario 1 is required, all other scenarios are option/recommended. And even within the scenarios there are optional items, e.g. phase specfiic power.

Also I am wondering you only added setters, but maybe it is also helpful to have getters to e.g. validate the current storage?