hdf5_ep

[svn-r25082] Bug HDFFV-8625, HDFFV-8639, HDFFV-8745:
    h5diff issues "unable to read attribute" error message when comparing
    two files

Description:
    h5diff did not handle attributes of variable length strings
    correctly. It created the wrong memory type if one but not both
    compared attributes are of the variable strings type. It ended in
    H5Aread() failure.

Solution:
    We decided to treat the case when one type is variable length string
    and the other is not as incomparable. That bypassed the above error.
    We still need to figure out the proper way, if possible, to compare
    variable string with non-variable string types.

Tested: h5committested plus tested in Swallow (OS X 10.8).