Overtone cp.m

Cross-polarization overtone experiment. Syntax:

          spectrum=overtone_cp(spin_system,parameters,H,R,K)

where H is the hamiltonian commutation superoperator, R is the relaxation superoperator and K is the chemical kinetics superoperator. The following parameters are required:

   parameters.sweep           vector with two elements giving
                              the spectrum frequency extents
                              in Hz around the overtone frequency

   parameters.npoints         number of points in the spectrum

   parameters.rho0            initial state

   parameters.coil            detection state

   parameters.Nx
   parameters.Ny              X and Y Zeeman operators on the
                              quadrupolar nucleus

   parameters.Hx
   parameters.Hy              X and Y Zeeman operators on the
                              spin-1/2 nucleus

   parameters.rf_frq          spin-lock frequency offset from
                              the overtone frequency on the 
                              quadrupolar nucleus, Hz

   parameters.rf_pwr          a vector of spin-lock powers on
                              the quadrupolar nucleus (first
                              element) and the spin-1/2 nucleus
                              (second element), Hz

   parameters.rf_dur          spin-lock pulse duration, seconds

Relaxation must be present in the system dynamics, or the matrix inversion operation below would fail to converge. The relaxation matrix should *not* be thermalized.