Added amount unit type

The amount unit has two types, 'Quantity' and 'Mole'. Quantity has been added so you can define a custom quantity without a specific type. E.g. lets say you have a product specification like contents which can either be a weight, a volume or just some sort of an amount (like pills, tablets or whatever). Now you can still benefit from storing the type of the unit in the same field as you can for the values that do contain a valid SI type.