Expose `MIN_MEASUREMENT_PERIOD` as a setting

[Flix6x]

It would be good to expose MIN_MEASUREMENT_PERIOD as a config variable or at least document it. A follow-up issue?

Originally posted by @nhoening in #108 (comment)

I would suggest renaming it and exposing it as an initialization option of the FlexMeasuresClient class or, probably more appropriate, the CEM class.