Function to parse date and timerange in iso 8601 format

Dates and time ranges will be required to be given in iso 8601 format in index definition yaml (reference period, see #273 (closed)) and for command line arguments (reference period and computational period, see #289 (closed)).

The end date of a time range should be interpreted as the upper bound of the date, i.e. 1961/1990 should mean 1961-01-01T00:00:00 to 1991-01-01T00:00:00 and 1961-01/1990-01 should mean 1961-01-01T00:00:00 to 1990-02-01T00:00:00 (see #273 (comment 34689))

The time range parser should have support for duration as start or end, i.e. P20Y/2100 should produce same as 2081/2100.