ridge-classbayes: Support distributions more generically.

Allow brightness and strength to be modelled by FA, GA0
or log-normal distributions.
Three fixes for ridge-classbayes:

1) Correct expression for F_A PDF.
2) Correctly swab likelihood values.
3) Use 4th root of strength value rather than using value directly.
Sort log likelihoods before summing them.

When calculating log likelihood ratios for point-based line classification, sort
point log likelihoods in ascending order before summing them to avoid any
possibility of floating point math issues for lines with very large numbers of
ridge points.