hdf5_ep

[svn-r15521] #1184

Add a userblock to an HDF5 file during the repack.  The user gives  
give a filename and userblock size as command line parameters to  
h5repack and the contents of that file are stored in the  
userblock for the HDF5 file created by h5repack.

New flags to handle this -u and -b
Tested : windows, linux