Clean up color legend labels

Please fix the locations of the color legend tick labels. If possible, they should be at round numbers, such as 2 or 1000 or 3e6 or 3.2. Currently, they are randomly located, such as 3.245654 or -43.98987.

From a discussion with Ken (that I totally agree with): "fix the damn positions of the major ticks. They are awful right now (and they used to be pretty amazing)."

"I'm not at my desktop to make a screenshot, but I'll try to describe freehand. Open disk_out_ref and color by Temp. Look at the major tick labels. They are at stupid positions. Humans need nice simple numbers to estimate interpolated values. The major ticks should be at 300, 600, and 900 or something like that. Instead, they are at worthless goofy positions like 682.54 or something like that."

Be sure to test with negative and positive numbers, numbers that are marge and small exponentials (a million in one test case, a millionth other test case). Be sure to test log scaling.