Add money (currency) type

[sirex]

Add money data type. money data type would store amount and currency, original amount and currency should be stored, but also a normalized value should be stored for comparisons and other money related operations.

Example:

d	r	b	m	property	type	ref
			Person
				salary	money	EUR

When property.ref is not given, then multiple currencies can be stored. In this case, money should be stored not as a single number, but number and currency together. If property.ref is given, then only amount is stored.

• _amount - returns money amount.
• _currency - returns currency (tree letter code).