Re-adds documentation on immutability of DataGroup's data layout.
Re-adds documentation on immutability of DataGroup's data layout.

The const-ness of the entries is actually a function of NamedData, not
of DataGroup, but the user of DataGroup should be made aware of that
immutability.

simquest/opensurgsim,simquest/opensurgsim,simquest/opensurgsim,simquest/opensurgsim