Add "at" keyword for imshow and other enhancements

[mcflugen]

This pull request addresses #1424 but adding an at keyword to imshow when used as a ModelGrid method. There are also a couple other changes:

• imshow is now available to all ModelGrids
• imshow can now guess at a location to plot values if the at keyword is not given. This allows things like: grid.imshow(values) and grid.imshow("topographic__elevation").
• deprecates the values_at keyword
• deprecates usages like imshow("node", "topographic__elevation")

[mcflugen]

@vaga3461, @ethan-pierce: I noticed you used the imshow_grid function a lot in your landlab clinic. Would one of you have time to give this pull request a look over and let me know what you think of the changes?

I added an imshow method to all of the grids (raster, hex, radial, etc.) so that you don't have to import imshow_grid anymore. So instead of,

from landlab.plot import imshow_grid
imshow_grid(grid, ...)

you can just do,

grid.imshow(...)

I've also made the syntax (I think) a little cleaner and more like other parts of landlab. Instead of,

imshow_grid(grid, "node", "topographic__elevation")

you can do,

grid.imshow("topographic__elevation", at="node")

If you don't provide an at location, imshow_grid will guess at a location.

grid.imshow(values)  # values is a array of values, doesn't have to be a field
grid.imshow("topographic__elevation"). # looks for the named field first at nodes, and then at cells

[vaga3461]

these changes look good to me, I like that you've made the syntax more similar to other functions. Thanks!

[mcflugen]

Thanks @vaga3461! I'll update the usages of imshow_grid in the notebooks and then merge.