Extend store open params schema to allow date limits

[pont-us]

Currently, the JSON schemas returned by the get_open_data_params_schema methods of data opener plugins (defined in xcube/core/store/accessor.py) can't specify an allowed range of dates. The returned schema can contain something like:

            time_range=JsonArraySchema(
                items=[JsonStringSchema(format='date-time'),
                       JsonStringSchema(format='date-time', nullable=True)]),

However, this gives no indication as to the supported range of dates. Unfortunately, JSON Schema does not allow "minimum" and "maximum" parameters to be specified for a date string (see json-schema-org/json-schema-spec#99 and json-schema-org/json-schema-spec#116 ). However, we could add an unofficial extension by adding (for example) optional min_datetime and max_datetime arguments to the JsonStringSchema constructor. The automatic JSON validation would of course not be able to deal with these, but they could be stored in fields for users of get_open_data_params_schema (e.g. UI builders).

[pont-us]

Closed by PR #336 .