h5fortran

HDF5 string character data is handled more broadly and robustly including:

• null pad character data as typically generated from HDF5 programs in C / C++ / Python / Matlab etc. are now read correctly
• variable length strings are read correctly (c null term)

Also, the common annoyance of a non-existing HDF5 file is handled with a succinct error message now.

fixes #26

Thanks @gekowa