Support Unity builds by adding a header guard and changing a few function names

[jmcarcell]

See Unity builds. To enable unity builds just add -DCMAKE_UNITY_BUILD=ON to the cmake command.

Some timings on my machine, only one run, building without optimizations and with Ninja:

Task	Normal	Unity
Build EDM4hep (with add_subdirectory)	22.4 s	8 s
Build podio+EDM4hep (with add_subdirectory)	47.5 s	20.3 s
Touch Frame.h and rebuild (podio+EDM4hep)	10.8 s	12.4 s
Touch ROOTReader.cc and rebuild (podio+EDM4hep)	1.6 s	3.9 s
Touch read_events.h and rebuild (EDM4hep)	1.4 s	1.4 s

Not great for development but interesting for builds from scratch at little cost, one include guard and different names for some functions, which may be better for debugging and setting breakpoints.

BEGINRELEASENOTES

• Support Unity builds Add missing include guards and make sure some functions such as createBuffer have a different name for each collection.

ENDRELEASENOTES