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
Fix moved Pandas class

Closes #383
Variables dragged into terminal take default label from .label attribute

Closes #392
Mergeable datasets

  - Merge method on DataCollection class
  - Merge action item in layer tree widget
  - Automerge suggestions on interactive load
Document data merging
Merge pull request #393 from ChrisBeaumont/pandas

Fix moved pandas class
Merge pull request #394 from ChrisBeaumont/terminal_label

Better default terminal variable names
Improve merge interface
Merge pull request #396 from ChrisBeaumont/merge

Allow dataset merging
ENH: specify hot keys for the majority of menu items

Allows for more convenient menu exploration/navigation/execution
Merge pull request #404 from yarikoptic/master

ENH: specify hot keys for the majority of menu items
Remove & from dict key (introduced in e252af49e47e73e0a84f1e52fb664177ca0f7dfa)
Add MaskSubsetState class

With the subset.state_as_mask method, any subset can be converted
into essentially a lookup table. This makes it easier to propagate
subsets to other datasets in some situations (for example spatially
overlapping datasets)
Add Menu item for maskifying subsets
Suppress error log when opening an image widget with no image.

Closes #402.
Take advantage of pandas parsing utilities

Now that pandas is a formal dependency, we might as well use it.

Closes #378