Update permission settings when installing

[weaverba137]

This PR:

• Closes Allow desiInstall to install packages with world-read #192.
• Closes Ensure that very old IERS files do not trigger an error #194.
• Adds test coverage to other aspects of desiInstall.

Summarizing changes to desiInstall:

• Module files will always be installed world-read.
• Software files will be installed world-read, and user,group-read-only, unless:
  • --no-world is specified, in which case world-read is disabled.
  • A branch is installed, including main, in which case user-write is enabled.
• Internally, fix_permissions.sh is no longer used. In fact, no external scripts or unix commands are used to change permissions.

[coveralls]

Coverage: 74.392% (+0.6%) from 73.826% when pulling 94c1025 on install-world into a15f4f9 on main.

[sbailey]

Looks good, thanks. I empirically tested that the new desiInstall behavior works as advertised (and that it matches what we intend for sharing code installations publicly).