allow for thinned series

[d2hydro]

Some series requests can only be make with thinning as the data density is too high in FEWS. So:

• include an array of thinned_series in config.json to specify if series are to be thinned
• include that option in the time series request
• test on rioolgemalen Aa en Maas to see if it works

[d2hydro]

Won't do for now:

• overloading FEWS-API with large amount of (smaller) calls
• need un-thinned data for downloading any way

We prefer:

• Alternative fast downloading of unthinned tss from FEWS (e.g. NetCDF)
• Alternatively; implement timeseries caching on our side