code: public BigMoney plusRetainScale(BigMoneyProvider moneyToAdd, RoundingMode roundingMode) {
          BigMoney toAdd = checkCurrencyEqual(moneyToAdd);
          return plusRetainScale(toAdd.getAmount(), roundingMode);
      }
docs: Returns a copy of this monetary value with the amount in the same currency added
  retaining the scale by rounding the result.
  <p>
  The scale of the result will be the same as the scale of this instance.
  For example,'USD 25.95' plus 'USD 3.021' gives 'USD 28.97' with most rounding modes.
  <p>
  This instance is immutable and unaffected by this method.
  
  @param moneyToAdd  the monetary value to add, not null
  @param roundingMode  the rounding mode to use to adjust the scale, not null
  @return the new instance with the input amount added, never null