QOL improvement: symmetric range button

[zarzycki]

When analyzing files containing anomalies (ex: things that come out of ncdiff or are files including climate "deltas") it is useful to use a white-in-the-middle colormap (e.g., bluered) with a symmetric range (e.g., -N to +N).

Currently ncvis selects a min/max range -- it would be helpful to have a little button next to the range icons that automatically sets the range to be -N to N where N = max(abs(currentmin),abs(currentmax))

Ex., if the default range is -0.3 to 4.0, clicking on "symmetric range" would automatically set the range to -4.0 to 4.0 without needing to type anything in.

ncview has this and I find it fairly useful.