324: Price Base should be Price Modifier Base and should be optional.

Changed definition of Price Base to Prices in today's power markets are often communicated other than as a simple price. The Price Base,is a low level abstract type which is an element in many other types. Price Base is an extensible type whose extensions include not only a simple or absolute price, but other types that rely on foreknowledge and computation. Unless otherwise specified (as it is in TeMIX which is restricted to only the simple price), wherever an information model requires a Price Base, any type derived from Price Base is supported