Initial version of genfit-ebuild.

Genfit ( http://genfit.sourceforge.net/Main.html ) is a generic track-fitting toolkit under LGPLv3. 

I have written a basic ebuild using their svn-tag of the current version v02-00-00. 
It can also be used together with RAVE ( https://rave.hepforge.org/ ), but this is not in portage and I have no experience with this yet (see elog message). 
A sub-slot dependency on sci-physics/root / dev-libs/boost may be required (c.f. https://bugs.gentoo.org/show_bug.cgi?id=505358 ), I can add this as soon as I find out that it is necessary. 

Looking at the build-system, the bundled version of the boost-headers in their repo are only used if boost is not found, so it should not be required to patch them out explicitly.