Fixed a nasty rounding bug in the cartography that was causing
noticeable discontinuities in the output images. Adjusted terrain
generation a bit as well, including reducing the period of ptrf (this
turned out to be unnecessary, but it's not a bad change IMO).

-Peter Mawhorter
Worked on the cartography so I don't have to constantly hack it to map
different variables. As part of that introduce the "gradient" struct as
a simpler alternative to gradient maps.

-Peter Mawhorter