Permit a saner structure for YAML files

[Maurizio Tomasi]

Currently the command importyaml requires that entities, quantities, and data files be specified in three different parts of the file:

entities:
  - …

quantities:
  - …

data_files:
  - …

However, these three concepts are usually nested, according to the sequence entity1→entity2→quantity→data_file. As an alternative to the approach shown above (which should not be removed), the importyaml command should accept this structure as well:

entities:
  - name: entity1
    - quantities:
      - name: quantity1
        data_files:
          - name: data_file1