Create a standalone hydraulic lookup table for channel cross sections

[kumdonoaa]

The creation of hydraulic lookup tables for channel cross sections, using either RouteLink's synthetic channel cross section data or topobathy data, currently occurs within the diffusive.f90 of t-route. To dynamically exchange flow or depth values computed among the MC, diffusive, and reservoir modules within t-route at each time step during runtime, the diffusive module needs to run at the same time step as the MC or reservoir modules, which is currently every 5 minutes. Running the diffusive kernel every 5 minutes unnecessarily repeats the creation of the lookup tables, which is only needed once. Therefore, this PR proposes creating an extension module (chxsec_lookuptable.cpython-<...>.so) that can be called to compute the lookup tables only at the initial simulation time of the diffusive routing and then passed to the diffusive kernel as input arguments at every subsequent simulation time step.

** A refactored version of diffusive.f90 to sync with this extension module is on the way.

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Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Visually tested in supported browsers and devices (see checklist below 👇)
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
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Testing checklist

Target Environment support

• Windows
• Linux
• Browser

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• Keyboard friendly
• Screen reader friendly

Other

• Is useable without CSS
• Is useable without JS
• Flexible from small to large screens
• No linting errors or warnings
• JavaScript tests are passing