flags query parameter accepts multiple occurrences

Currently, the data endpoint only accepts a single value for the flags query parameter. This single value can contain multiple different values separated by comma but if a user decides to go with the form of using multiple key-value pairs instead only the last value will be used.

Example:

https://toar-data.fz-juelich.de/api/v2/data/timeseries/18763?flags=AllOK,AllQuestionable -> will return all data points with either flag
https://toar-data.fz-juelich.de/api/v2/data/timeseries/18763?flags=AllOK&flags=AllQuestionable -> will return all data points with flag AllQuestionable

Both these examples should work like the first one.