Improve p4estindent workflow

[cburstedde]

Description

We have the p4estindent script and would make its use safer and more standardized.

Proposed solution

• Put logic into p4estindent that makes it fail when working directory and git index are not the same.
• Add a CI job to run p4estindent separately for each source and header file and report missing indentation.

[lcw]

For what it is worth, my current version of the git pre-commit hook that checks to see if a file is indented is here https://github.com/lcw/p4est_occa_tutorial/blob/084c089328819c5c9ebdb6ae90507bfc2a379152/misc/git/hooks/pre-commit

[cburstedde]

That could be a nice starting point.