Recommended way of using providers

[asnyv]

We should consider if providers generally should only be initialized with the raw data it reads, and not e.g. a filtered or resampled version. An example is the EnsembleSummaryProvider which may be initialized with a pre-sampling to e.g. monthly frequency. If you plugin / plugin instance uses monthly, one raw, one yearly and so on, we have the same data set in several forms stored. Might be better to always store as raw, and rather later resample / filter, similar to what is done today in SimulationTimeSeries if sampling is not defined?