Support index files to accelerate reading HDF5 metadata

[Barry Wardell]

Carpet can write an index file which contains all the metadata (but not the data) from its HDF5 files. This file can be read much faster than reading the metadata from the original data files. h5mma should support using these files.

[Ian Hinder]

This is something which could probably be handled at a higher level (e.g. in SimulationTools). The index files are identical to the original HDF5 files, except they have no data written to their datasets, so they are much smaller. As a result, the existing code in h5mma should be able to read any required metadata from the index files already without having additional code introduced. I think h5mma is probably the wrong place to add this functionality, as it is application-specific, not specific to reading HDF5 files in general.

[Barry Wardell]

I have created an issue for SimulationTools instead.