Diverging cmap

[eugenioLR]

New arguments have been added called diverging and vmaxabs. They were added to the following functions: _SetupImage, imgplot, imghist, filterplot and ImageGrid.

The argument diverging forces vmin to be equal to -vmax, ensuring that diverging colormaps are displayed correctly, mapping the middle value to 0. The argument vmaxabs sets vmax to vmaxabs and vmin to -vmaxabs.

I added documentation and unit tests to everything that was needed.

This implements the features discussed in issue #697.

[SarthakJariwala]

Thanks for putting this together. I left a couple of comments that will have some minor changes throughout the PR but I didn't end up commenting everywhere.

Let me know if you have any questions.

[eugenioLR]

I think that would solve the issues you raised. Thank you for your feedback.

[SarthakJariwala]

Thanks again for working on this - looks good to me! I've left a few minor suggestions.

Lastly, can you add an example in the Examples section of the docstring in imgplot. It will help with feature discovery. If you would like to do it as a separate PR, I'm open to that too.

[eugenioLR]

I adressed the changes you proposed, which seemed reasonable. The stuff with the comments was a mistake, it's now solved.

I also added the example in the docstring of imgplot, it didn't seem to be very hard at all, so it's not an issue. The only thing i worry about is if the example is meaningful enough, but it shows the feature so I think it's ok. Thanks again for you feedback.

[SarthakJariwala]

The example looks good for now. Thanks for working on this.

I'll release it as part of v0.9.0