[enh] #111: create compute_amounts() function

* Uses a list of user amounts and a multiplicator (should be CENT_MULT_TO_UNIT or UD_Value).
* Multiplies the given amounts(UD) to get usable int() amounts for Duniter.
* Makes sure that each amountUD is superior to the minimal amount.
    * Absolute amounts are in FloatRange(MINIMAL_AMOUNT), so they have already been checked.
    * This check should be updated when solving issue #306 / DUBP v13.
* Returns list of amounts.