Rename compile guide .doc files to .dox
Remove class overview image

Not everyone has GLE to build it. Besides, it is probably out of date by now.

Also fix issues building the dev docs.
Fix compiling with ENABLE_CUDA=off
Remove unnecessary check if body < 0

This silences a compiler warning on clang.
Unit tests compile again

Now to figure out why they all seg fault
Removed unused variable

Was causing a compiler warning