The Cressman scheme is implemented!

I believe I have successfully implemented the Cressman
distance-dependent weighting scheme. I also successfully implemented a
"top of the atmosphere" check, which helps to speed up processing time
by removing radar range gates that are extremely far. This is controlled
with the "toa" parameter similar to the original mapping code. Note that
I also plan to extend this to radius of influence (ROI) considerations,
whereby we remove radar range gates that will never be captured within
the ROI of any analysis grid point. This commit also includes some minor
improvements in code structure, e.g. removing redundant calculations.