Fix output units for index function count_occurrences

It seems that the index function count_occurrences expect output to have units day. This is however the proposed_unit, which not consistent with current the current canonical unit 1 as defined by the standard names. This has probably remained under the radar because I was until recently using a really old version of index_definition.yml. If simple to fix then target bugfix milestone 13.2, else postpone it to a later milestone.