[reporting] Convert the units of the input data to a target unit.

[victorgarcia98]

Description

In order to make reporters more reusable, we need to get the data in a known unit. For example, if we want to have a reporter to compute the P&L, we need the units to work out, for example:

P&L [EUR] = Price [EUR/MWh] x Energy [MWh]

Before, we expect the sensors to have certain units which made the reporter unusable for sensors with equivalent (but different) units. Moreover, I hope this new feature helps to improve the readability of reporter configurations.

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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

[victorgarcia98]

Indeed, this PR lacked a test. Other than that, I also added the changelog entry.

[victorgarcia98]

Added the unit conversion from in the output stage. Please, let me know what you think. :D