Updated install instructions and makefile

[SnowHydrology]

Additions

• Installation instructions (link to INSTALL.md from README.md)

Changes

• Removed cheyenne directory and replaced with relative path in makefile
• Changed confusing "calib" and "pgf" names in makefile

Testing

1. Built and ran model on MacOS following new install instructions with gfortran

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 :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

• [ ] Linux
• [ ] MacOS