#119 - ISO 8601 inputs

This is part of the work for #119 and addresses the [ISO 8601 wiki page](https://github.com/cylc/cylc/wiki/ISO-8601).

It uses an ISO 8601 Python library for parsing and manipulating date/ time representations from https://github.com/metomi/isodatetime and implements some bridging code in cylc to implement our custom abbreviations and formatting. There is a fairly good range of tests.

This should allow us to start testing within cylc.

@hjoliver, please review.