XIOS: Throw an error for non-positive read or write periods

[joewallwork]

XIOS: Throw an error for non-positive read or write periods

Fixes #971

Task List

• Linked an issue above that captures the requirements of this PR
• Defined the tests that specify a complete and functioning change
• Implemented the source code change that satisfies the tests
• Commented all code so that it can be understood without additional context
• No new warnings are generated or they are mentioned below
• The documentation has been updated (or an issue has been created to do so)
• Relevant labels (e.g., enhancement, bug) have been applied to this PR
• This change conforms to the conventions described in the README

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Change Description

I tried out XIOS with zero restart period and it stalled so figured we should throw a warning to stop users encountering this.

I also noticed that 0 is handled in a special way, implying that the period is the full length of the simulation.

As such, I've made it so that 0 gets handled in this way consistently and anything negative throws an error.

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Test Description

n/a

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Documentation Impact

The XIOS docs page has been updated to reflect this.