timeDimension.localRepresentation should be optional

[hoehrmann]

https://github.com/sdmx-twg/sdmx-json/blob/71fe5eaa9fcd29e3c15f2f0216a19b9b650b1dbd/structure-message/docs/1-sdmx-json-field-guide.md#timeDimension and the JSON Schema currently require timeDimension.localRepresentation with the only required sub-property being format.dataType. Among the allowed values for that is ObservationalTimePeriod which encompasses all other allowed values. ObservationalTimePeriod should be the implied default value there, and specification of it should be optional.

(In doubt, please handle this as a public review comment on SDMX 3.1 once the comment period begins.)