Fix auto-centering of altitude colormaps.

[juseg]

I realize the documentation are dotted with center=False. This is because xarray defaults to centering any image plot where data span negative and positive values, however this is not really appropriate for glacier-depressed bedrock elevation typically ranging from slightly below sea level to the mountain tops. I can think of two ways to fix this: use sealevel=-120 and possibly the 'Elevational' instead of 'Topographic' colormap, or changing bedrock_altitude() and friends' default behaviour.

• [ ] ~Implement better defaults for altitude?~
• [ ] ~Remove center=False in docs.~
• [x] Doc changes in whatsnew.

[juseg]

I am backtracking on this. I think I want to keep hyoga as consistent as possible with xarray. The center kwarg is not the most intuitive though. Besides I realized that sealevel=sealevel is redundant with center=sealevel. So I am planning to:

• [x] replace center=False with vmin=0 ~or vmin=-120~ in docs.
• [x] deprecate sealevel argument to bedrock_altitude.

[juseg]

Using vmin=-120 alone causes xarray to center the cmap between -120 and +120, so I'll stick to center=False in that case but:

• [x] add a note to explain center=False in docs.