fix: allow showing beliefs for sensors with non-unique names

[Flix6x]

Description

• Internally, sensor objects rather than sensor names are now used as keys in a dict that holds data for multiple sensors.
• IDs are automatically shown (together with a warning) in case of sensors with non-unique names.
• It is no longer possible to accidentally pass the same sensor ID multiple times when calling flexmeasures show beliefs.
• In flexmeasures show beliefs, IDs can be toggled to always show in legend labels and column headers using the new flag --include-ids.

How to test

Trigger an error by passing ID 2 twice:

flexmeasures show beliefs --sensor-id 1 --sensor-id 2 --sensor-id 2 --timezone Europe/Amsterdam --start 2022-11-01T00:00+01 --duration P1D --to-file temp.csv

Include IDs in the plot's legend labels and the file's column headers, only in case of non-unique sensor names:

flexmeasures show beliefs --sensor-id 1 --sensor-id 2 --sensor-id 3 --timezone Europe/Amsterdam --start 2022-11-01T00:00+01 --duration P1D --to-file temp.csv

Always include IDs in labels and headers:

flexmeasures show beliefs --sensor-id 1 --sensor-id 2 --sensor-id 3 --timezone Europe/Amsterdam --start 2022-11-01T00:00+01 --duration P1D --to-file temp.csv --include-ids

Related Items

Closes #867.

This has the potential to break other things, though, so we should be careful in deprecating the use of sensor names as dict keys (consider e.g. that adding another option to search_beliefs to toggle the new behaviour is safer than modifying existing function arguments or return values).

I checked the use of sensor names as dict keys, and found it only to be used by legacy FM code that has already been removed (analytics page, for example). Only 1 FM plugin that I know off is relying on this feature, for which I'll make a separate PR.

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