improve PIMixin for Ensemble Data

[SGeeversAtVortech]

In GitLab by @MaartenS on Feb 22, 2019, 15:10

There are basically two issues with the current PIMixin:

• it does not read ensembleMemberId when an ensembleId is mentioned, only ensembleMemberIndex. As one cannot solve this on the FEWS side through ID mapping (and it probably not being wise to allow this), we should make sure that RTC can ingest ensemble data where either ensembleMemberId or ensembleMemberIndex is used, and not only the latter.
• minor problem: upon import, RTC gives numbers to the ensembleMemberIndex and does not create a dictionary to keep track of this mapping, for example for writing outputs, or internal use in for example pre() to do something with known traces.