h5 : add get_triqs_hdf5_data_scheme for vector<T>

-was done only for vector of string
- NB : the actual hdf5 serizalisation of vector<double>
is the default string -> to be changed to a proper hdf5 array
... not urgent.
improve time_pt

- make_beta

- privatize constructor from double
to avoid catastrophic error when using it to construct beta
(because of rounding error, it could something give 0 instead of beta).