Fermion/Boson statistic uses old-style enum #989

It is possible for the following to compile (obviously wrong, but an easy typo to make):

auto w_mesh = mesh::refreq{beta, triqs::gfs::Fermion, n_w};

because triqs::gfs::Fermion is taken as an int -> double.

As commented by @parcollet, this kind of thing might be avoided by changing:
enum statistic_enum { Boson = 0, Fermion = 1 };
to
enum class statistic_enum { Boson = 0, Fermion = 1 };

but this would also be a somewhat irritating update because Fermion -> statistic_enum::Fermion.

Opening an issue not to suggest it for an immediate change, but to register this as something to keep in consideration.