Gene masks (like for plant genes) used unpadded num2hex on a random
number 0-255. This had the (technically ok) result of masks for values
1-15 being a single character but also had the (not ok) result of the
mask for value 0 being the empty string. This only happened to 1/256 of
genes, so was easy to miss.
This change pads all of them to 2 characters so this issue won't happen
and to line things up nice.
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