Add attributes for File-based TimeSeries
I was thinking that it might be useful to add one or two fields to the file-based TimeSeries classes, i.e.
CompressionTypeValue could be either an enumeration or a codelist.
The attributes could be added to both classes, or an abstract class _TimeSeriesFile could be created between them and AbstractTimeSeries. In the latter case, other shared existing attributes (uom, file, etc...) could be moved to _TimeSeriesFile.