Added more specifics about software versions, etc. to README.
Fixed backwards-characteristic integration! The negative initial value of s(r)
was the culprit. Turns out we don't even need to "pad" this initial value -
setting it to machine-precision zero works fine.

Also I think it's probably a bit conflicting to have the reference to a variable
also given the "const" property (specifically GridClass) so I removed the const
attribute and changed GridClass to a regular pointer in calc_rays.