Line-of-sight sampling of discrete clouds implemented. Seems to work
well, as long as number of clouds is <=4 (higher numbers may work,
but have not yet been tested), and number of samplers per dimension
is high (50 seems to work). Run times are significantly shorter
than for other extinction parameterization (~1 sec/star total on
laptop w/ dual-core i5, hyperthreaded to 4 virtual cores).