Pre-Compiler vs. Macros

[ax3l]

Regarding ComputationalRadiationPhysics/picongpu#1693 it looks odd to force #pragma calls to also start at the 0 column in a file.

#pragma, besides #pragma once, are not necessary pre-compiler instructions that make sense to be aligned that way but are non-C++-standard, extended type descriptions (attributes, no-inline, etc.) and non-C++-standard, compiler-specific compile-time hints.

[ax3l]

Full list of actual pre-compiler commands this rule was thought for:

#include
#define
#undef
#if
#ifdef
#ifndef
#else
#elif
#endif
#error
#line
// [add more!]

[ax3l]

offline discussion with @psychocoderHPC : accepted for the reasoning above.

For now, even if it is not a 100% correct match but simpler, our scripts will simply skip lines including a #pragma rule.
(Not all #pragma calls are compiler hints but most, e.g., only #pragma once is a pre-compiler instruction.)

PR to update this style guide is welcome!