Tiny changing the way in which toString is calculated

via ESTATIO_OBJECT_CONTRACTS.
Using IsisConfigurationForJdoTests (ISIS-754) in EstatioSystemInitializer.
Unit tests of AgreementRoleCommunicationChannelType (nb: fix to matchingCommunicationChannel())

This fix is a change in behaviour to production code.  It's possible (though very unlikely) that this fix could break stuff elsewhere; the class is not critical to the invoice calculation, however.