Redo metadata aggregation

[berland]

In #182 , the metadata aggregation for summary vectors will be redone to make use of ecl2df's new feature for metadata

ecl2df.summary puts the needed meta-dictionary into dframe.attrs["meta"] for all summary keys returned.

ensemble.py should aggregate this metadata for all realizations and present this using the same attrs on returned dataframes.

The API get_smry_meta() will be less important, as meta is always returned.

Suggested changes to fmu.ensembles:

• {realization/ensemble}.get_smry_meta() requires summary data to be preloaded, it will not go to disk to obtain requested summary data from libecl, only look through the attrs["meta"] in its internalized frameds
• the column_keys argument to get_smry_meta() is dropped, as it seems unnecessary to code slicing of aggregated dictionaries on users behalf.