Expanded the CR72 model to handle both the R20A and R20B parameters.
The 'CR72 red' model now functions as the old CR72 model where R20A = R20B = R20. All parts of the
code have been modified. The lib.dispersion.cr72.r2eff_CR72() function has been expanded to support
the full Carver and Richards 1972 equations, dropping back to the simplified form if R20A = R20B.