Clarify/Fix NumberValue calculation for conversions

[atsticks]

IMO the current number value used as round factor does not honor any MathContext/capabilities of the target amount:

In spi.AbstractCurrencyConversion:

protected NumberValue roundFactor(MonetaryAmount amount, NumberValue factor) should optionally round the factor to be used. By default this method will only round

• as much as it is needed, so the factor can be handled by the target amount instance based on its
• numeric capabilities. Rounding is applied only if amount.getContext().getMaxScale() > 0 as follows:
  • If the amount provides a MathContext as context property this is used.
  • If the amount provides a RoundingMode, this is used (default is RoundingMode.HALF_EVEN).
  • By default the scale used is scale of the conversion factor. If the acmount allows a higher scale based on amount.getContext().getMaxScale(), this higher scale is used.

Please leave your comments, if this should create any problems. You may also test the changes done with the latest master version of the repo.