Updated the Bjorkman & Wood method to use continuous absorption.
A few more aesthetic changes.
More minor aesthetic changes.
Consolidated propagate_photon_bw and propagate_photon_lucy into one
function, propagate_photon_full, and moved it to the Grid class.
Streamlined a number of functions, updated many of the CartesianGrid
functions to make them work better, added some comments...
A few more changes to remove unnecessary code from CartesianGrid.
Last modification to CartesianGrid (hopefully...) to finish the current
round of updates.