mc_tools : improve treatment of infinite mc ratios

- when the ratio returned by an attempt of a move is infinite,
previous code was just throwing TRIQS_RUNTIME_ERROR.

- Now when the ratio is infinite, it is replaced by a large number
  (>1 is enough for metropolis), and the sign is properly updated
  using std::signbit.

- NB :

- a double/float in C++ can be :  normal/ zero/ nan/ infinite / subnormal.
  Here, the code will recover only from infinite case.

- std::signbit works for infinite (according to standard).
Merge remote-tracking branch 'triqs_lib/master'