Add units of measure classes. Convert length node to using length class.
Begin stubbing out volume and area classes.
Progress on area and volume.
Add a settings menu for setting units.
Update display units when units preference has changed.
Remove old measurement classes.
Add area node.
Fix to area from string conversions
Add area settings to the menu.
Add operator overloads to unit types and custom logic in evaluators for math operations to handle units.
Make formula use units by removing the units.
Cleanup.
Move unit nodes to separate file. Fix fractional inch formatting.
Revive unit tests for SIUnits.
Remove equality stuff...for now.
Merge branch 'master' into units
added Is Null? node
added basic null value error notification
Add Volume. Reorganization.
Better exceptions for units.