Possibility to allocate with custom rounding parameter

[Crovitche-1623]

In Switerland, 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