fixed a bug in uv_selector

[SaulAryehKohn]

In auxiliary scripts that call aipy.scripting.uv_select (e.g. capo/scripts/pull_antpols.py) there was a problem that you required a list of antennae, and could not just select-off a polarization without an error occuring. This was due to the way the loops were nested in the aipy.scripting.uv_select function, leading to an undefined variable "pol" if you hadn't looped through antennae first. I've corrected this issue by using the variable "pol_str", which one enters into the function with.

[AaronParsons]

I'm pretty sure this is merged in already with HERA-team, so I'm closing this pull request.