Changing forcing_code to external submodule

[WanruWu-NOAA]

Changes to be committed:

new file:   .gitmodules
new file:   extern/README.md
new file:   extern/forcing_code/

deleted:    forcing_code/include/aorc.h
deleted:    forcing_code/include/aorc_tools.h
deleted:    forcing_code/include/bmi_aorc.h
deleted:    forcing_code/src/aorc.c
deleted:    forcing_code/src/bmi_aorc.c

modified:   make_and_run_pass_forcings.sh
modified:   src/main_pass_forcing.c

Additions

• Adding extern/forcing_code/
• Adding .gitmodules
• Adding extern/README.md

Removals

• Deleting forcing_code/

Changes

• Modifying forcing_code path in make_and_run_pass_forcings.sh
• Modifying forcing_code path in src/main_pass_forcing.c

Testing

• Done (including remaking PET, rebuilding ngen, rerunning PET parameter testing and comparing results)

Screenshots

Notes

Todos

• To modify extern/README.md

Checklist

• [x] PR has an informative and human-readable title
• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [x] Code follows project standards (link if applicable)
• [x] Passes all existing automated tests
• [x] 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 :point_down:)
• [ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• [ ] Reviewers requested with the Reviewers tool :arrow_right:

Testing checklist

Target Environment support

• [ ] Windows
• [ ] Linux
• [ ] Browser

Accessibility

• [ ] 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