hdf5

[svn-r6990] Purpose:
    Bug fix and minor code enhancement

Description:
    Missing methods to read/write C++ String for an attribute and
    a dataset.

Solution:
    Added overloaded functions read and write to H5::Attribute and
    H5::DataSet.

    Also, added another constructor StrType so the need to separately
    set the length of the string type can be eliminated.  It's minor
    but convenient.

    Made some minor changes to make error messages more readable.

Platforms:
    SunOS 5.7 (arabica)
    Linux 2.4 (eirene)
    IRIX 6.5.11 (modi4)
    HPUX 11.00 (kelgia)