feat: add AggregatorReporter

[victorgarcia98]

Description

This PR adds the class AggregatorReporter to compute the aggregation (SUM or MEAN) of different sources' into a single one. This reporter allows to add the aggregate energy flows, costs, emissions, etc... of a system in a time period (e.g hourly).

Details:

• This reporter makes source a required parameter in beliefs_search_configs.
• Supports passing weights for the different alias. This can be used to convert a consumption (in absolute value) to a negative flow or to convert a current into power (i.e $P = V \cdot I$ for biphasic and $P = V \cdot I \cdot cos(\theta) \cdot \sqrt{3}$ for triphasic).
• The (optional) parameter method can be set to SUM or MEAN to chose the aggregation method.

Look & Feel

{
    "beliefs_search_configs": [
        {
            "sensor": 1,
            "source" : 1,
            "alias" : "PV"
        },
        {
            "sensor": 1,
            "source" : 2,
            "alias" : "CONSUMPTION"
        }
    ],
    "method" : "SUM",
    "weights" : {
        "PV" : 1.0,
        "CONSUMPTION" : -1.0
    }
}

Testing Steps

Test for MEAN and SUM cases in flexmeasures/data/models/reporting/tests/test_aggregator.py.

[Flix6x]

Very nice to have. Are you sure you don't want this to subclass from PandasReporter instead?

[Flix6x]

Looks like this can even be shipped with 0.14. I added a changelog and moved the relevant entry to the Infrastructure / Support category. I also added an experimental disclaimer, because we are planning to still refactor these classes soon (see #662).

[nhoening]

So can this be merged?

[Flix6x]

Yes, but let's give @victorgarcia98 a chance to review my commits first.

[victorgarcia98]

Thanks! Nice the addition of the experimental warning.