parse_gctx slicing issue

[rajivnarayan]

#2239: "Cannot use parse_gctx on level 5 data from GEO"
Slicing a subset fails with
"Error using containers.Map/subsref
The specified key is not present in this container.

Error in parse_gctx (line 122)
ds.h5name = attr('name');

Error in cmapm.Pipeline.parse_gctx (line 45)
ds = parse_gctx(dsfile, varargin{:});
"