Added persistence for GrassCoverErosionInwardFailureMechanism. Added tests check for UpdateConversionCollector so that only existing objects (those with a ID other than 0) can be marked as updated. Related to Issue [WTI-367] git-svn-id: https://repos.deltares.nl/repos/WettelijkToetsInstrumentarium/trunk@3072 5a3db67b-de53-47b5-99c8-a1c30a6650e2