Doxygen tweaks

Some improvements for Doxygen

* Running `make doc` now works when you're in the `build` subdirectory. Docs are now written to the `build/docs/` folder.
* Some helpful graphs are created by default if graphviz is installed
* Option to also create caller/callee graphs (`cmake -DDOC_FULLGRAPHS=YES ..`)
* Increased vertical space by removing the header which just contained the project name