add wrapper to isolate all HDF5 API calls in the ch5md library

Calling HDF5 API calls from main LAMMPS creates problems on installations where HDF5 has been configured to use the v1.6 API by default (e.g. ubuntu 12.04 LTS).
This change adds a wrapper for the call to the ch5md library, where it is much easier to override the default with preprocessors directives.
It is the cleaner approach anyway.