[reporting] Droplevels pandas reporter

[victorgarcia98]

Description

Reporters can have input sensors with different sources, for example, one power sensor with a source of type Scheduler and another one of type User. This makes simple operations like summing their values fail. For this reason, we prepended most of our reporters with an instruction to drop all indexes but event_start. This PR introduces a little feature to automate this process with a new flag to control its behavior.

Related Items

https://github.com/FlexMeasures/flexmeasures/issues/869

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• [X] I agree to contribute to the project under Apache 2 License.
• [X] To the best of my knowledge, the proposed patch is not based on code under GPL or other license that is incompatible with FlexMeasures