Add HDF5 file service

This adds a very simple service that holds a pointer to an HDF5 file. It allows multiple algorithms or tools to write datasets into the same file. There are no clients (so far) in Athena, but I've implemented similar code in a few frameworks and having standardization would help.

The interface, IH5GroupSvc, provides a pointer to an H5::Group object, which is the base class of H5::H5File and has a sufficient interface for most clients. The only concrete instance so far returns an H5::H5File when the group() is requested, but it seemed safer to introduce the less flexible base class interface first.