Improving consistency of include guards

[cflaviu]

The include guards in project's headers are either classic #ifndef-#define-#endif guard or #pragma once. Only one type of include guard should be used.

[ArsenArsen]

The latter is preferred and should be used in new code, as far as I understand.

+1 on making this consistent (clang-tidy rules perhaps?)

[cflaviu]

I prefer #pragma once too despite the fact I don't know when this include guard may fail.

[Dennisbonke]

#pragma once is in fact preferred, we settled this in the coding standards document found here.

[cflaviu]

PR created