Updated CMake for inclusion of REST via find_package

[lobis]

Changes to the CMake files to support inclusion of REST via find_package(REST).

Added new command: rest-config --prefix which returns the installation path of REST (doesn't have to be the same as the current REST_PATH).

When doing find_package(REST) it will check that the current REST_PATH and the installation path of REST match. It will throw an error if they don't.

[lobis]

relevant PR: rest-for-physics/restG4#33

[jgalan]

Ok, but how could it happen that rest-config --prefix is not the same as REST_PATH?

[lobis]

Ok, but how could it happen that rest-config --prefix is not the same as REST_PATH?

The user could have changed the REST_PATH env variable after sourcing REST, rest-config --prefix is safer in general, in a similar way to root-config --prefix vs $ROOTSYS.