performance improvements for R HDF5 module

[Marcus G. Daniels]

Thanks to Hugh Pumphrey (Dept. of Meteorology, The University of
Edinburgh), the HDF5 plugin is now much faster, esp. for HDF5 files that
have large, multidimensional data sets.

  ftp://ftp.swarm.org/pub/swarm/src/testing/hdf5_1.4.6.tar.gz

Other features include a flag to `hdf5load' to convert key names to be
R friendly (specifically, convert all non-alpha or "." to "." in order
to avoid variable names that could be confused with operators), and
support for Fortran style strings (non-null terminated).  Both are
thanks to Hugh.

In addition, there's a bug fix for a nasty endian problem with boolean vectors
that shows up on big endian machines (e.g. Sparc).

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