Create new `PandasReporter` to consider Pandas methods

[victorgarcia98]

In the discussion https://github.com/FlexMeasures/flexmeasures/discussions/662, we resolved (https://github.com/FlexMeasures/flexmeasures/discussions/662#discussioncomment-5803785) to rename the PandasReporter to TimelyBeliefsReporter and create a new reporter named PandasReporter which doesn't consider any timely-belief specific functionality in the transformations: beliefs, probabilities, sources, etc...

Following the renaming of PandasReporter to TimelyBeliefsReporter (Issue https://github.com/FlexMeasures/flexmeasures/issues/828), we need to create the new PandasReporter to consider only Pandas specific methods.

In summary, this requires to work on:

• [ ] a new schema which validates that only pandas methods are used in the transformations
• [ ] _compute_report simplifies the belief dataframes before applying the transformations