setup Sand boost for refactor
(Sand).bought to .totalDug

redo last replacement of Molpy.sandDug to (Sand).bought
see:
https://github.com/eternaldensity/Sandcastle-Builder/commit/6f3645b5af3d7cda9be1464027bac008a6264770

should fix any errors (none known) and prevent future ones
remove Molpy.sandManual

replaced with (Sand).manualDug
migrate Molpy.sandPermNP to (Sand).sandPermNP
migrate Molpy.computedSandPerClick to (Sand).sandPerClick
remove checks for (Sand).spent == undefined

no longer needed since property is created on boost creation
remove old comment