Add drop=True argument to isel, sel and squeeze

[shoyer]

Fixes#242

This is useful for getting rid of extraneous scalar variables that arise from
indexing, and in particular will resolve an issue for optional indexes:
#1017 (comment)

[jhamman]

This looks quite nice. Just expand the tests for squeeze a bit and we're good to go.

[jhamman]

Can you add a few more tests to this section:

• squeeze empty array
• squeeze 2d array with a single element

[shoyer]

done

[shoyer]

Going to merge this shortly unless anyone has objections

[pwolfram]

@shoyer, I think this is really great and appreciate you doing this awesome work. I obviously have a vested interest in the drop=True capability because we have found great use for it via where. My vote is go for it!

[shoyer]

I obviously have a vested interest in the drop=True capability because we have found great use for it via where.

Well, to be clear drop=True works pretty differently on these methods than in where, so this might be confusing.

For isel, sel and squeeze, it drops scalar coordinates variables that would arise from indexing, whereas for where it drops some coordinate values. That said, the drop method already handles both cases.