Enforce project wide #include order

[pjungkamp]

This instructs clang-format to enforce a uniform #include order across all files. See https://clang.llvm.org/docs/ClangFormatStyleOptions.html#includecategories

1. ^<villas/: headers
2. ^<[[:lower:]]+>$: standard library headers
3. ^<.*>$: other system headers
4. ^".*"$: local headers

[n-eiling]

Didn't we want to keep the clang format file as simple and as standard as possible? Is this really worth it?

[pjungkamp]

I've been asked to reorder includes in multiple PRs, about half the files in this repository contain some form of error regarding our current std - other - villas header order.

I just want clang-format to sort the includes for me instead of getting a "please reorder includes" review in every second PR.

The enforced order is of cause up for debate, if you want to stick with std - other - villas.

[n-eiling]

Who asks you to reorder? You should not need to do anything style related that is not mentioned here: https://villas.fein-aachen.org/docs/node/development/contributing/. This mentions nothing about include order.
From the code-perspective include order shouldn't matter?

[stv0g]

I think we agreed in the past to follow the LLVM/Clang coding standards.

The minimal clang-format configuraiton which we had in the past is not sufficient, as it can not identify our local headers (e.g. #include <villas/...).

So, believe extending the clang-format config is justified as long as we stick with our goal of following the Clang/LLVM coding standard.

[pjungkamp]

The LLVM #include style can be found here: https://llvm.org/docs/CodingStandards.html#include-style

I deviate from these on the "main" header of a source file. clang-format has built-in heuristics to determine a source file's "main" header. But these work best when project includes use quotes instead of angle brackets for local headers.

I've found that these heuristics determined e.g. <sys/socket.h> as the "main" include for clients/opal-rt/rtlab-asyncip/models/send_receive/src/socket.c, so I didn't enable it in this PR.

[IgnoreWarnings]

I've been asked to reorder includes in multiple PRs, about half the files in this repository contain some form of error regarding our current std - other - villas header order.

I just want clang-format to sort the includes for me instead of getting a "please reorder includes" review in every second PR.

The enforced order is of cause up for debate, if you want to stick with std - other - villas.

Great idea to automate/standardize to reduce PR review workload and frustrating comments.

[stv0g]

We have a new pending PR by @pjungkamp to automate include ordering fixes with clang-tidy. Looking forward to this