Tweak header guard names #45
Description
Any header guard of the form __ABCD_H__ is reserved in Linux.
If we change our header guard names from __ABCD_H__ to PHYSICC_ABCD_H (or PHYSICC_ABCD_HPP), then we completely eliminate the (somewhat minute) chances of a header guard name clash, and make it easier to determine what project the header guards belong to.
We can even name the Light header guards to LIGHT_ABCD_H to minimize the friction when we pull the Physicc code out of the Light repo and make it into its own submodule.
This kind of naming scheme is not new, and is in use in large projects like Eigen.