This replaces header include guards with `#pragma once`.
A couple of include guards are not removed yet (e.g. `__RNA_TYPES_H__`),
because they are used in other places.
This patch has been generated by P1561 followed by `make format`.
Differential Revision: https://developer.blender.org/D8466
Function names will be updated in a separate commit.
This will be the place for the new particle system and other
code related to the Simulation data block. We don't want
to have all that code in blenkernel.
Approved by brecht.
