Read forcings only when necessary

[joewallwork]

Read forcings only when necessary

Fixes #1054

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

Currently, if the ERA5 and TOPAZ modules are included then they are read every timestep. This is unnecessary given that ERA5 data are only available once per hour and TOPAZ once per day.

This PR makes it so that ERA5 forcings are only read at the top of the hour and TOPAZ forcings are only read at midnight.

Caveats

1. Assumes that the simulation begins at midnight.
2. Assumes that timesteps are aligned with the hour.

Are we happy with these assumptions? If so, I can add config checks to ensure that they are satisfied. If not then I will have to modify how the XIOS calendar is handled.

(This change is also required to enable ERA5 and TOPAZ forcings with XIOS.)

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Test Description

XIOS tests are updated accordingly.

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Documentation Impact

A note on the assumptions has been added to the XIOS doc page.

[timspainNERSC]

This will be fine. There is an assumption that the time step length will either evenly divide an hour or be a multiple number of hours that evenly divide the day. But this seems to be a reasonable assumption.

I'd recommend not printing the time string every time you want to check for fresh data, though.

[timspainNERSC]

To avoid string parsing every time, I suggest getting the duration since the epoch in seconds and seeing whether it is the top of the hour:
(tst.start - TimePoint()).seconds() % 3600. == 0.
(This should work: if it doesn't that's a bug in the Time classes).

[joewallwork]

Done in 74d7891. Similarly for TOPAZ in ac44e60.

[joewallwork]

[Rebased on top of develop after merging #1051]

[TomMelt]

Thanks @joewallwork . It looks good to me

[timspainNERSC]

Looking good, now