Functions to parse dates and durations in iso 8601 format (fixes #294)

I've implemented functions to parse dates and time range (time intervals) in iso8601. So far the code only explicitly handles calendar dates (i.e. no specific consideration is given to hour/minute/second parts). The regular expressions to check the date format do not check for strict compliance to iso8601, since I'm relying on the isodate package to report any errors regarding e.g. invalid months or day numbers.

I've added test functions for the currently handled formats.

Also added isodate package as requirement/dependency.

Keeping as draft just to get some feedback from #273 (closed) before finalizing.