Reference period definition in index_definitions.yml

To conclude after offline conversation. Some index definitions utilises a reference period in the definition for the calculation and there is no obvious way in how this should be handled. A suggestion would be to included the reference period in the clix-meta index definition, given the following example:

    index_function:
      name: count_thresholded_percentile_occurrences
      parameters:
        data_threshold:
          kind: quantity
          standard_name: lwe_precipitation_rate
          long_name: "Wet day threshold"
          data: 1
          units: "mm day-1"
        data_condition:
          kind: operator
          operator: ">="
        percentile:
          kind: quantity
          standard_name:
          proposed_standard_name: quantile
          long_name: "Percentile value"
          data: "{PRC}"
          units: "%"
        percentile_condition:
          kind: operator
          operator: ">"
        reference_period:
          kind: time_period
          data: ['1961-01-01 00:00:00', '1991-01-01 00:00:00']

the reference period could then be used in the index calculation and the output should contain a global attribute giving information about which reference period that was used.

global attributes:
		:reference_period = "1961-01-01 00:00:00 to 1991-01-01 00:00:00"

or something similar.

Notes there were also a discussion about having an additional Auxiliary coordinate variable with the following information:

reference_period:(var_name: "reference_period", standard_name: "reference_epoch", value: mean(n1,n2), bounds: [n1, n2], unit: "days since 1961-01-01 00:00:00")

where n1 and n2 are the point values for the start and end of the time period defined by the unit.