Class to specify a cashflow, i.e. the outcome of either of computation (e.g. interest accrual) or an assessment of some sort (e.g. a fee). The cashflow can then be turned into a cash transfer, artefact to be used as the input to a payment system or the outcome of it. The associated globalKey denotes the ability to associate a hash value to the Cashflow instantiations for the purpose of model cross-referencing, in support of functionality such as the event effect and the lineage.