Updated user guide, especially Developers section
Fixed small problems with random numbers; moved GaussRand to new generators
1. - Added new tree cell type parameter
2. - Moved some functions from TreeMonteCarlo to KDRadiationTree
1. - Added 'jerk' for SPH particles for star interactions (only used currently for computing time step if required)
2. - Tidied up some comments at various places
Fixed turbulent velocity field generation so the larger modes are preserved for higher grid resolution