Fail hard when multiple DATA files found

[berland]

How fmu-ensemble will react when it finds multiple DATA/UNSMRY-files in a directory is ambiguous, but it only gives out a warning.

The suggestion is to fail hard from fmu.ensemble 2.0.

ScratchRealization.init() provides a find_files argument in the concurrency branch. This should be available also in the ensemble init to allow API users to specify the ECLBASE.

At ensemble level, the ECLBASE must be possible to supply using globbing, as the realization index can be involved at the realization level.