Dissipation exchanges

[bkuczenski]

get these working. basically inherit from ExchangeValue, except the value is the fractional dissipation, and there's an additional attribute which is the flow property to use.
These need the following requirements:

• process may have only one reference flow, and that reference flow must be characterized by the named property in order to use the dissipation exchange (otherwise, defaults to the static value?)
• this implies that the emissions cannot be allocated-- thus avoiding the multiple-inheritance problem
• most drudging part is serializing / deserializing dissipation exchanges
• hardest part (yet unsolved) is how to deal with parameterization