Allow addition / subtraction of default MonetaryAmount with any other (neutral element)

[sjanel]

This has one main benefit: containers of MonetaryAmount can be initialized with default MonetaryAmounts that will be involved in additions / subtraction later on.
Example:

vector<MonetaryAmount> v(10); // 10 default MonetaryAmount
v[0] += MonetaryAmount(10, "EUR");
v[1] += MonetaryAmount(2000, "KRW");