Prevented to take log of negative values, and division by zero.
This though slows the implementation down.
Systemtest Relax_disp.test_baldwin_synthetic_full went from 6.x seconds to 8-9.x seconds.
sr #3154: (https://gna.org/support/?3154) Implementation of Baldwin (2014) B14 model - 2-site exact solution model for all time scales.
This follows the tutorial for adding relaxation dispersion models at:
http://wiki.nmr-relax.com/Tutorial_for_adding_relaxation_dispersion_models_to_relax#Debugging
