Add contour map method

[dstansby]

I'm doing some contouring at the moment, and thought a helper function might be helpful. My current use case is contouring AIA 193 maps to isolate coronal holes, and then plotting those contours on other maps.

Essentially this just uses skimage.measure.contours, but returns the result as SkyCoord objects. Here is what the example I have added to the gallery looks like:

Needs

• A documentation example
• What's new
• Tests

Test ideas:
Sample maps with:

• No contours
• One contour

[dpshelio]

I think it's very useful!! Whether in the future we want these contours as RoIs and be able to extract properties based on them on other datasets is something we could discuss beyond this PR. For now, having this available simple as it's now I think is good. 👍 from me!

[hayesla]

I think just a small test is now needed

[ehsteve]

I feel like this provides 90% of the functionality of compositemap. I am not complaining! Just another reason to potentially remove compositemap. Ping @Cadair.

[mbobra]

This is awesome!!!

[dstansby]

Thanks! Since this won't make it in to 2.0 anyway, I'm waiting until I have a bit more time to add tests.

[nabobalis]

This looks good to me, but I thought we (cadair) were against adding more to the map API?

[dstansby]

I think methods are more pythonic than functions, but happy to go either way if someone wants to make a decision.