Begin work on PV Extraction
Proper intensity scaling with NaNs
half pixel fix
Tweak DS9Normalize.__call__

Needed for MPL colorbars to work properly
Better interaction with PV slice widget

Visual Indicators for connection between slice and parent data
Crosshair tweak. Updates on drag only
Docstrings
Slice icon
show/hide slice mode with data dimensionality
Non-editable data viewer selection
Fix sampling bug in PV extraction
Sync pvextractor
Sync pvextractor
Remove pvextractor's internal tests

Causing Travis segfaults
Fix failing test
Merge pull request #301 from ChrisBeaumont/pv

PV Slice Extraction
WIP wcs axes
Pull in wcsaxes
Fix test failures
Extension parsing logic

Closes #304