Calculate grays; use NaN for unknown.
Calculate grays; use NaN for unknown.

Instead of 0 we now use None for unknown entries in python.
We translate None to javascript's NaN. This marks more clearly which
entries are actually unknown, in case any are zero.

Also we now calculate grays by averaging diametrically opposite colors
on the wheel, since the renotation data from RIT doesn't include these
grays (chroma = 0). The HTML5 page has not yet been updated for this.

pteromys/munsell,pteromys/munsell,pteromys/munsell