Simplify trace_ray, remove heap mallocs

[jobh]

First, thanks for a very nice and usable library.

In my effort to understand the trace_ray algorithm, I rewrote it a bit to remove explicit heap allocations. In my opinion, the result is a bit simpler, so it might be worthwhile to merge.

Briefly, the ray std::vector now stores coordinates rather than pointers to coordinate triples. A memoryview is used to alias the active/last point in ray.

It is probably a bit faster. I think there is also a slight difference in behaviour in that the endpoint itself is checked for NaN value, whereas before only the first step was checked.

[malcolmw]

Thanks for your contributions @jobh, and I am glad you find the package useful! I am merging this for testing now.