Add support for spacetime

[nuest]

• put out STIDF (Irregular (long table)) in getDataAsST
• getObservation(..) returns still a data.frame, internal structure we know
• option
  • if we're requesting features first (GetFOI), and then timeseries, then we can also put out full (STFDF, all timeseries measure at the same point in time) or sparse datasets (STSDF), use xts::merge(..) which fills up missing values