Add generic Aggregator processor

[danielhuppmann]

This PR is in relation to a request by @jkikstra for a CEDS-emissions-processor that renames and aggregates emissions-variables from IAM results to match the CEDS schema - see the related PR at IAMconsortium/common-definitions#255

The idea is to have a mapping like

- Target Code:
    - Variable A
    - Variable B

that can then be called like

ceds_mapping = AggregationMapping.from_yaml("ceds-mapping/ceds-variable-mapping.yaml")
ceds_aggregator = Aggregator(dimension="variable", mapping=ceds_mapping)

ceds_aggregator.apply(df)

to apply the aggregation-mapping onto an IamDataFrame df.

The method uses the pyam rename() method because that actually works (with summation) on all dimensions. This could be extended later for different aggregation-methods, etc.

This could be useful for @lisahligono as part of the scenario-transfer-routine, where the rename-mapping for regions is implemented as an external yaml file and then just called like shown above as part of the processing-and-transfer.

[danielhuppmann]

Yes, I will still add tests but wanted to hear any comments about feature-design first...

[phackstock]

As per bilateral discussion, I'd say this is a good way forward.
I'd include the dimension attribute in the yaml file.

[dc-almeida]

Makes sense to me!