CultureInventoryType does not support UnitType #239

In CultureAdditionType, we support measurement by VolumeType or MassType or UnitType. The latter is useful for small-scale brewers who measure yeast (be it dry or liquid) by "number of packets".

In CultureInventoryType however, we only support measurement by VolumeType or MassType. Specifically, we have separate measurements for each of the following:

• liquid: VolumeType
• dry: MassType
• slant: VolumeType
• culture: VolumeType

There are two problems with this. Firstly, it prevents the user from measuring by UnitType, even though that's allowed in CultureAdditionType. Secondly, it repeats/contradicts CultureBase.form, which already specifies whether the culture is liquid, dry, slant, culture or dregs. So, eg, if you have a culture that is dry, why would you want to be able to measure four different amounts for it (liquid, dry, slant and culture).

I think both these problems could be solved by changing CultureInventoryType to have a single field, amount, which is the same as CultureAdditionType.amount. This would bring it into line with Fermentable, Hop and Misc.