CountableFeatures: support for unitary prices

[Aerendir]

The configuration should be something like this:

a_countable_feature:
    type: countable
    cumulable: true
    unitary_price:
        EUR:
            monthly: 1000
            yearly: 10000
        packs:
            ...

Find a way to differentiate the forms. Maybe adding a suffix like _packs and _units may work.

This will break the setting of the default value.

Then is necessary to find a way to calculate the right instantPrice and to save the amount of units bought. More, is necessary to decide what happens in case there are packs set.

For example, if exists a pack with 50 units and the user buys 55, the 50 units should be paid following the packs price or the unitary_price? The remaining 5 units will be paid for sure following the unitary_price.