Added ROOT to CMakeLists.txt and added the AmBe neutron spectrum from ISO to a ROOT histogram in AmBe.root.
Moved AmBe.root to the inputs folder. Fixed a bug with the geometry where a particle would get stuck between the cylindrical volumes because they were constructed incorrectly - the outer cylinder still needs to have an inner radius of 0.
Neutron energy is now determined for each event from the spectrum using the ROOT TH1 GetRandom function, which pulls a value from the distribution depending on its frequency.