Add support for listed paths for each ensemble?

`fmu-ensemble` supports a list of paths instead of a single path. Can consider to allow for list of paths to increase flexibility in which realizations that are included in an ensemble.

Locate the fmu-ensemble entry point and trace where a single path is accepted and used to select realizations. Check for existing tests covering path selection, then define done as accepting multiple paths while retaining current single-path behavior and passing the relevant test suite.

Written by the indexing model from the issue text.