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