XIOS: read all inputs

[joewallwork]

XIOS: read all inputs

Towards #1056

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

This PR avoids the need for an XiosInput configuration section. Instead, XIOS reads all field variables that it finds in any input files whose names are provided.

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Test Description

XIOS tests are updated to account for this change. Some tweaks were required to avoid the same variable name (u) being read from both restarts and forcings.

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Documentation Impact

The XIOS docs page has been update accordingly.