Update isodatetime

[MetRonnie]

Describe exactly what you would like to see in an upcoming release

Update the version of isodatetime used in cylc-flow once metomi/isodatetime#176 is done and 2.1 is released. This should give performance improvements for datetime cycling workflows (see metomi/isodatetime#162). Also, if metomi/isodatetime#80 gets fixed that vanquishes a long-standing bug that is in cylc-flow.

Optimistically tagged against 8.0rc1 as it would be good to have done before then, but not a priority. See discussion below.

Pull requests welcome!

[MetRonnie]

@oliver-sanders @hjoliver I have this tagged against 8.x, but seeing as there is a breaking change (metomi/isodatetime#183), would this be be better suited to 8.0.0?

[oliver-sanders]

The breaking change shouldn't be a problem (Cylc is used as a system rather than a library), good to keep 8.0.0 to the more mission-critical aspects right now.

[MetRonnie]

Forgot to mention, we could just release isodatetime 2.1.0 now and finish metomi/isodatetime#176 later in 2.1.1. Upgrading cylc-flow to isodatetime 2.1.0 shouldn't take too long.

[MetRonnie]

Optimistically tagged against 8.0rc1 as it would be good to have done before then, but not a priority