tuple tools : simplify

- simplify implementation of several functions.
- clean API, rename, etc...
- add more documentation
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.
     Also the infamous mpi+ hdf5_cpp bug, for which we have no clean solution.

- clean the notion of parent of a group. Not needed, better iterate function in C LT API.
- modified doc : no need for C++ bindings any more.
- modified cmake to avoid requiring CPP bindings.