Get all dataset variables in the DDXIngester

[aperrin66]

Harvesting dataset variable from DDX metadata works only for a specific metadata structure and for standard variable names accessible through pythesint. It would be nice to have a more generic mechanism.

To do:

• extract all variable names from the DDX metadata (in DDXIngester._extract_attributes())
• process variable names which are unknown from pythesint (add them as wkv? create them on the fly?)