POST:metadata must create MetaDatumRecords for all MetaDataSets

[lkuchenb]

Data schema integrity requires that all MetaDataSets have exactly one MetaDatumRecord per MetaDatum associated. This is currently violated during metadata addition / modification. The following changes have to be implemented:

• POST:metadata must create corresponding MetaDatumRecords for all metadatasets for the new metadatum. If the metadatum is mandatory, a default value that passes the specified constraints must be specified
• DELETE:metadata must remove all corresponding MetaDatumRecords (maybe that's already cascaded by SQLalchemy)
• PUT:metadata must take into account the same constraints as POST

Service MetaDatumRecords have to be filled with defaults only for those metadatasets which have a corresponding service execution.