multiple steps:
- the functions for inquiring datasets and attributes do not return
an HDF5 type any more but an enum of type h5_types_t. This change
was required for the Python module.
- bugfix in reading attributes: See https://git.psi.ch/H5hut/src/issues/4
- several consts and macros have been moved from the public C-API to
the core API
- more consitent file naming
- several 'private' function have been moved to their 'private' header
files as 'static inline'.
- minor formatting changes
