Allow units to be rounded automatically on purchase list

[moseschmiedel]

Provide setting for unit that it should snap to specific values on the purchase list.
For example the unit Piece makes only sense with whole numbers, so it should be automatically rounded on the purchase list.
Maybe we also want to implement a step function that specifies which values are only valid for a given unit, so one can specify a step function: x = x + 1000 so that the unit just has values divisible by 1000.

[dboehmer]

Maybe we should have a column unit.step which defines the step for the <input> elements. Then we could have buttons to increase/decrease to next step or round mathematically on all/selected purchase list items.