Use new isodatetime copy methods

[MetRonnie]

Needs:

• Create a public .copy() method that supports changing properties metomi/isodatetime#209 addressed + a corresponding isodatetime release

We currently have at least 1 case of mutating isodatetime data classes, however they are supposed to be immutable. We should switch over to the proposed .copy() methods that will allow changing of atttributes. See #4554 (comment)