inclusion for cdgroups (and semantics of nested) cdgroups.

[kohlhase]

In #57 proposes to add a cdgroup attribute to OMOBJ as a catalog for default cdbases. This works well, if we can build "the cdgroup of all things I care about". This would be made much much easier, if CDGroup had an inclusion mechahism.

Concretely, I propose to add an element CDGroupInclude with a href attribute that specifies the URI of the CDGroup to be included.

Acting on these inclusions would create "nested CDGroup", which we could flatten into an un-"nested CDGroup", which constitutes the meaning of the inclusion-extended CDGroup.

[kohlhase]

But if we want to use CDGroups as catalogs as suggested in #57, then

1. we need CDURL children of CDGroupMember to be present (or defaulted).
2. we need to require uniqueness (see there seems to be no uniqueness requirement for CD names in a CDGroup #60 for a discussions).

For 1. I would suggest that if a CDURL is not present, then its URL is derived from the CDGroup URL by replacing the file name with the value of the CDName field and appending the .cdg extension.

For 2. I propose that the last (we could equivalently also use the first, but last is more often used I think) URL for the CDName determines the catalog CDURL.

[kohlhase]

I have included this into #61.