Add C++ code Formatting

[mabruzzo]

Pull request summary

This PR adds a support for using clang-format with Enzo-E.
I have added extensive documentation.

Detailed Description

This is something we spoken at length about in the past and there is consensus enforcing standard formatting for C/C++ code would be beneficial.

I feel like this is something we could delay indefinitely. So I made 2 particular choices to make this easier to merge:

1. To avoid merge-conflict I have excluded just about all existing files from clang-format.
   • If we don't do this we will be trapped in a cycle of waiting for PRs to be closed (which never ends)
   • I think there's value in applying formatting new files and "ripping the band-aid off" later for the older files
2. I decided to "just make a decision" about code-formatting.
   • This is something that we could endlessly bikeshed, and I think it would be beneficial to choose now.
   • I basically choose the Google C++ style and made a few very minor adjustments based on a few different choices made in the LLVM style (the two styles are extremely similar). I have put forth a similar style in Grackle.

• I have reformatted a small subset of existing files to show what the style looks like. I'm happy to make changes if you see something egregious.
• For the most part, I think we should leave iteration on minor details to a later date. That's the beauty of code-formatting.

Relationship with pre-commit

The main challenge of using clang-format is that it is not forward/backward compatible (picking an established style should mitigate some of these issues). - In other words, you need to apply a consistent version of clang-format everywhere.

• Since clang-format is part of the LLVM project, this can be somewhat non-trivial to enforce (it could be very inconvenient to start requiring people to use).
• If we didn't take any steps to address this problem, it could be a large inconvenience.

SOLUTION: To address this challenge, this PR configures the pre-commit software and pre-commit.ci continuous integration service (introduced in #419) to invoke clang-format.

• Essentially, you will be able to modify the formatting of proposed changes in a PR without locally installing any software by leaving a comment that says "pre-commit.ci autofix" to a PR (then a commit will be added to your branch to fix the problem).
• To easily run clang-format locally, I recommend installing the pre-commit software on your machine and invoking that software. The pre-commit software will download its own personal copy of clang-format to use for formatting (it won't affect the rest of your system)
• I have added extensive documentation about how this works (and how to do it).

I implemented a similar solution in Cholla and it works well.

This is a major change or addition (that needs 2 reviewers): unknown

Depends on #419

PR Checklist

• New features are documented with narrative docs.

[mabruzzo]

At the last Enzo-E call, it was suggested that I try to format everything and check if it works.

• A branch of the repository where everything is formatted can be found here.
• I confirmed that everything definitely compiles when we do that

[mabruzzo]

pre-commit.ci run

EDIT: For posterity, this comment was used to trigger the new pre-commit.ci continutous integration (this was necessary since I introduced precommit.ci after this PR was created and I didn't want to push a new commit)

[mabruzzo]

pre-commit.ci autofix