if chl <= chl_min, we already have log10(chl_min) stored as a parameter.
Avoiding the computation of log10(chl) in those cases prevents the possibility
of a floating point exception, and lets us replace a max() statement with an if
check so it shouldn't affect performance.

This requires adding chl_min to the optics_type (which already has log10chl_min
and log10chl_max).

previous commit only avoiding log10(0) in lookup_ohlmann_swpen() but we have
another log10(chl) in the opacity function