Colorbar and tick marks for symlog scale

[huiwanpnnl]

Hi @olearypatrick (and @jourdain ),

I tried visualizing a variable i_vmr_bc_a1_preAerMic which has very small values (about 1E-13 to 1E-10). When using a simlog scale combined with a discrete colormap:

• The colors shown in the color bar don't seem to match the colors in the image.
• There is often a "0" tickmark shown on the color bar between the min value and the tick mark of 10^-11. At first, I thought this "0" was really a zero, meaning that colors to the left of this tick mark corresponded to negative values. But this variable is positive definite, so I think the "0" is more of a number format issue. In data slices with lev index around 30, we can get two 0's for tick marks.

I used v2.6.1 at NERSC. Here is the state file: quickview-state_symlog_tickmark_and_colorbar.json