Update code base formatting

[davidscn]

Main changes of this PR

Upgrades the clang-format version to clang-format-12, introduces a column limit of 120 as discussed in #1155 and applies corresponding changes to the code base. Resolves #1155. In case anyone has formatting objections, now would be a good time to mention them.

Author's checklist

• I added a changelog file with make changelog if there are user-observable changes since the last release.
• I ran make format to ensure everything is formatted correctly.
• I sticked to C++14 features.
• I sticked to CMake version 3.16.3.
• I squashed / am about to squash all commits that should be seen as one.

Reviewers' checklist

• Does the changelog entry make sense? Is it formatted correctly?
• Do you understand the code changes?

[davidscn]

With this changes, clang-format resolves the current formatting ambiguity of our code base (e.g. both code versions would have been accepted in our current formatting settings). However, having multiple function arguments on multiple lines might be desirable. Do we want to enforce something line breaks in function arguments or go with the current version?

[fsimonis]

I prefer one item per line. I find that it makes the signature easier to digest.

[davidscn]

Yes I agree, these arguments

BinPackArguments: false
BinPackParameters: false

would set the arguments on separate lines if they don't fit into the 120 characters. Or do you want to enforce the separate line for all functions (cf the docs)?

[fsimonis]

Let's move the reformatting behind the 2.4.0 release to avoid breaking at least #1279 #1258 #1271.