annual indices based on part of the year are not seasonal

Currently they have to handled as seasonal to allow the season (i.e. relevant part of the year) to be specified. As result, the global variable frequency = "sem" rather than the correct frequency = "ann"

Could the code interpreting the metadata.yml entry

    period:
      allowed:
        annual:
        monthly:
        seasonal: JASOND
      default: seasonal

be changed to either output frequency = "ann" when there is just one entry (string) and not a list of strings, or to allow something like

    period:
      allowed:
        annual: JASOND
        monthly:
        seasonal:
      default: annual