Update/extend documentation of PrimitiveID class

This merge request

• re-writes and extends documentation text of PrimitiveID class
• adds three images to better explain the format of the bit pattern
• groups documentation of PrimitiveID::asIntArray() methods together
• updates meta-data in PrimitiveID.hpp
• relocates PrimitiveID.hpp to the sub-folder src/hyteg/primitives