ifast

use a Runge-Kutta routine without error control to solve the ray tracing equations and a look-up/interpolation routine to evaluate certain trigonometric functions; with ifast=0, a Runge-Kutta method with error control and the intrinsic trigonometric functions are used. Using ifast=1 is about 30-40% faster and provides essentially the same accuracy as ifast=0 (default: 1)

Note: ifast=1 is intended to be used for all routine modelling and ifast=0 only to test a final model or if ifast=1 obviously fails