Convert header guards to use `#pragma once`

[fevangelista]

Description

A brief description of the goals accomplished by this PR

User Notes

• [ ] Features added
• [ ] Changes to compilation (if any)

Checklist

• [ ] Added/updated tests of new features and included a reference output.ref file
• [ ] Removed comments in code and input files
• [ ] Documented source code
• [ ] Checked for redundant headers
• [ ] Checked for consistency in the formatting of the output file
• [ ] Documented new features in the manual
• [ ] Ready to go!