Move imshow closer to px pattern

[nicolaskruchten]

@emmanuelle here's the first pass on the PX-ification of the imshow function. I'm realizing that to align better with the other px methods we should probably add height, width and template and the rest of the px defaulting machinery...

[nicolaskruchten]

Here I've bolted on the applicable parts of the "defaults cascade" logic for which I added a test suite the other day :)

[nicolaskruchten]

This lets us do stuff like this:

[emmanuelle]

💃 when the CI is green. Which reminds me that we should add imshow examples to the percy test file, one with single-channel and one with RGB. If you don't want to add it here (I should have done this in the initial PR) I'll make another PR for this.

[nicolaskruchten]

Ah right yes... If you could do it that would be great :)

[nicolaskruchten]

Ah drat also we need to tweak the PR in py-docs