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
This data block will be the container for simulation node trees.
It will be used for the new particle node system (T73324).
The new data block has the type `ID_SIM`.
It is not visible to users and other developers by default yet.
To enable it, activate the cmake option `WITH_NEW_SIMULATION_TYPE`.
New simulation data blocks can be created by running `bpy.data.simulations.new("name")`.
Reviewers: brecht
Differential Revision: https://developer.blender.org/D7225
