[FhirPath] Add support for Quantity arithmetic

[ewoutkramer]

No description provided.

[marcovisserFurore]

We have to implement the following in Hl7.Fhir.ElementModel.Types.Quantity:

        public static bool operator +(Quantity a, Quantity b) => throw Error.NotSupported("Adding two quantites is not yet supported");
        public static bool operator -(Quantity a, Quantity b) => throw Error.NotSupported("Subtracting two quantites is not yet supported");

        public static bool operator *(Quantity a, Quantity b) => throw Error.NotSupported("Multiplying two quantites is not yet supported");
        public static bool operator /(Quantity a, Quantity b) => throw Error.NotSupported("Dividing two quantites is not yet supported");

The method TryCompareTo has already been implemented.

Steps for implementing the arithmetic operations:

• first try to canonicalize both of the quantities to align the units
• When the units are aligned, then do the arithmetic operation