Refactor PV slicer into plugin tool
Moved spectrum tool to plugins directory
Get SpectrumTool to work with new API
Fix tests
Use registry for plug-in tools
WIP
Remove print statement
Make ToolRegistry capable of distinguishing between different widget classes
Remove unneeded options
Fix 'if' logic for ToolRegistry
Fix test failure
Merge pull request #475 from astrofrog/pv-plugin

Factor out spectral cube and PV slicer tools as plugins