consider adding support for ISO date format: yyyy-mm-dd

[sgratzl]

it is is a common format for defining date data both for parameters and return values.

[sgratzl]

to discuss if we want to support the new date format:

• for ranges also a - dash is used. Thus a range with this iso format is difficult. it would be 2020-01-05-2020-02-07
• if we want to change the return date format, we would need a new parameter: which name?

[krivard]

yyyy-mm-dd:yyyy-mm-dd is probably suitable for ISO ranges, and we should enforce that each individual query use either all-ISO or all-integer dates so that the parse logic is simpler.

The clients handle parsing the return dates into datetimes, so there's no need to handle that at the server level.