Swenzel/next

Several key improvements:

a) we are able (for the first time) to compile VecGeom into a shared lib on Linux + MacOS and propagate this info correctly to the installation cmake config.

b) make Navigation code generator work again (needed to adapt to various changes that happened between March 2016 and now)

c) several improvements to navigation benchmarking / navigation code generation; Most notably: Script steering code generation and compilation as well as ability to load a created plug-in library and benchmark it;

d) some smaller things