0-rank arrays?

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We're developing an alternative straw-person draft of mapping netCDF
to HDF5, at least for the first deliverable of netCDF-3 on HDF5.

Currently, we're thinking of storing the netCDF dimension IDs for a
netCDF variable in an HDF5 array attribute, _ncdimids, associated with
the HDF5 dataset corresponding to the variable.  This works fine for
array variables, but scalar (rank 0) netCDF variables have no
associated dimension IDs.

Various ways to encode this in HDF5 include:

 - no _ncdimids attribute for scalar variables
 - an _ncdimids attribute that is not an array

 - a rank-0 _ndimids array attribute.  In this case, the rank of
   _ncdimids would always correspond to the rank of the associated
   variable, whether scalar or dimensioned.

The latter is not currently possible in HDF5, but would be if rank-0
arrays were supported.  

I don't know how disruptive it would be to permit rank-0 arrays as a
Datatype, so if it has lots of unpleasant side effects, please just
ignore this minor request.  But if it's relatively easy, you might
find other uses for rank-0 arrays, for example, as something to return
to represent an empty result of a query that typically returns an
array.  We've found that treating rank-k multidimensional arrays
pretty much the same for k = 0, 1, 2, ... makes some code simpler.


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