Possibility to allocate with custom rounding parameter

In Switzerland, the smallest coin is CHF 0.05. It would be a good new feature to be able to allocate the amount with a custom rounding parameter. For instance by adding a parameter to the `allocate` function: 
`$amount->allocate(3, 0.05)`

so the result would be : 

 `8 / 3 = 2.65 + 2.65 + 2.70`
