Fix ods datetime and time export and import

[pascalfree]

The ods format was not matching the spec (Table 14) on datetime and time:

• datetime must be handled like date: The value goes into the date-value attribute instead of the value attribute.
• time value must be formatted as a duration according to XML spec.

TODO

• Add backwards compatibility, to import the broken time format as it is exported in the current version?
• Add test for time import
• Add test for datetime export

[claudep]

Thanks! I guess I'll wait for the test before approving? You could also add an entry in HISTORY.md

[claudep]

Looks good, thanks!

[claudep]

Could you still rebase and squash, please?