Added ParamsClass constructor to initialize everything to zero.
This reverts part of bbfe530f3bacde074b4f46c29523fdcfd108a4c0, wherein
I removed the nearly-identical calc_rays procedures (one for forward
characteristics, one for backward) in favor of a single templated
routine. Unfortunately the two types of characteristics are different
enough (specifically, the backwards ones have negative s(r) by
definition) that a templated routine just won't work. So I've replaced
it with the old version containing the two nearly-identical routines
(differing only in a few minus signs).