hdf5 : clean up

- For users : only change is :
   H5::H5File in apps. to be replaced by triqs::h5::file, same API.
- using only the C API because :
   - it is cleaner, better documented, more examples.
   - it is the native hdf5 interface.
   - simplify the installation e.g. on mac. Indeed, hdf5 is
     usually installed without C++ interface, which is optional.
     E.g. EPD et al., brew by default.
- clean the notion of parent of a group. Not needed, better iterate
  function in C API.
- modified doc : no need for C++ bindings any more.
- modified cmake to avoid requiring CPP bindings.
Fix #85 : narrowing conversion