suite-state point format conversion fix

[hjoliver]

These changes close #3600:

Write cycle point format to the DB suite_params table if a format exists, even (in the ISO8601 case) if it is the default format. This allows cycle point conversion to work in the cylc suite-state command and the suite_state xtrigger function, if the upstream suite uses the default format but the downstream suite or command does not. (On master, we assumed that conversion is only necessary if the target suite uses a non-default format).

(Note I tried to write a unit test for this using CylcWorkflowTestCase in cylc/flow/tests/util.py but gave up when mocked objects got in the way of writing to the suite DB. The functional test is simple and easy to understand, at least).

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Appropriate tests are included (unit and/or functional).
• Appropriate change log entry included.
• No documentation update required (fix to behave as advertised).