Add DBLtoMenuItem.vi as a helper for MenuCol

Because users may want to allow variables to select items normally chosen
from a menu, DBLtoMenuItem converts a passed double value n (like from Lua's
Str2DBL.vi) to the round(n)th menu item text (0-indexed).  That is,
passing a value of 0.0 would get the top menu item, 1.0 the next one, etc.