extend discretize bath functionality

[the-hampel]

This PR adds a more advanced bath discretization function for triqs:

• add function make_delta to construct a hybridization function on the imaginary axis for given hoppings and bath energies
• changes to discretize_bath:
  • allow to fit BlockGf objects, with blocks of arbitrary size
  • allow for complex hybridization
  • fit is directly performed on iw or tau
  • add gradient based minimizer BFGS and add basin hopping
  • allow to pass either a complete guess for the hoppings and energies, or a single value for all hoppings

[Wentzell]

Just to remember, we wanted to improve on the initial guess for the V matrix by using directly lim_{iw->inf} iw * Delta(iw) or -Delta(0) - Delta(beta).

[Wentzell]

Thank you @the-hampel for these improvements!