some issues in class DvQuantity

[Sergio Miranda Freire]

It seems that it is not necessary to test for the type of precision in the initializer.

The invariant Precision_valid: precision >= -1 is not implemented. It should throw an exception if precision < -1.

The initializer lacks the magnitudeStatus argument and so does the superclass initializer.

The methods compare ("<") is not implemented.

The method isStrictlyComparable is not implemented correctly: it should query the measurement service for the equivalence of units and return true if the objects comparable are both DvQuantity and their units are equivalent.

The methods add, subtract and negate in DvAmount are not implemented.

It would be interesting to define a representation value for this class, wouldn't it?