fix bug in select for single string antenna_name, flatten higher-dimensional arrays in select

[bhazelton]

Fixes a bug where if a single antenna_name string was passed it treated the characters in the string as separate antenna names rather than using the whole string as the antenna name.

Also add asserts to prevent problems with nested lists selects.

[adampbeardsley]

I think asserting that ndim=1 is fine. Are there any use cases where people would supply larger dim arrays and we should just flatten?

[adampbeardsley]

@bhazelton I went ahead and approved this, but I'll leave it to you to merge pending what you think about my above comment.

[bhazelton]

Hmm. I guess we could just flatten them. The problem I was trying to fix was that the error you got with higher dimensionality was completely confusing, but flattening would fix that.

I guess the question is: is there any situation in which flattening is the wrong behavior? I'm struggling to think of one.

[adampbeardsley]

Yeah, I'm struggling to think of either a case where flattening is wrong, or an actual use case where higher dim arrays would be input. Either way, we should probably make the assertion error more clear. I think as is it just gives:
AssertionError:

[bhazelton]

Ok, maybe I'll just change it to flatten the arrays. Then I think I don't need the assert.

[bhazelton]

Does the flatten ordering matter?

[bhazelton]

@adampbeardsley I changed all the asserts to flattens. Can you take another look when you get a chance?

[adampbeardsley]

Sorry, I thought I approved the update before I left work yesterday. All looks good!