Range selections on scatter plot converted to single-attribute subsets.

Previously, subsets were defined using very long rectangles.

Fixes #379
Merge pull request #380 from ChrisBeaumont/379

Range selections on scatter plot converted to single-attribute subsets
Data Loaders for the table formats which astropy parses but does not autodetect.

This allows users to load such files from the GUI, without writing a wrapper
loader just to set the format kwarg
Skip astropy0.3 test on old astropy
Merge pull request #381 from ChrisBeaumont/new_ascii_formats

New data loaders for astropy ascii formats
Layouts don't overlap
Preset layouts
New layout snap algorithm in GUI