Dimension scales

[jjhelmus]

NetCDF4 files use HDF5's concept of "dimensional scales" to provide named and common dimensions to multidimensional arrays. h5py supports these scales through the Dataset.dims attribute. pyfive should support reading this information from HDF5 files and provide a similar API for accessing these labels.

I believe this would require implementing support for referenced datatypes (class 7) as well more complete support for variable length datatypes.