More robust unified fitting in Minuit.migrad and Minuit.mnprofile

[HDembinski]

Closes #1008

This patch was developed in collaboration with
ikrommyd.

• Minuit.migrad: When we have to iterate, MnSimplex is now called by
  default in between interations, since this seems to improve convergence
  in pathological cases. The new keyword use_simplex can be used to
  disable this feature. It is only used if the algorithm has to iterate,
  so for well-behaved functions there is no penalty of extra function
  calls. Also, when we have to iterate, the strategy is now automatically
  increased to 2, since this also seems to help.
• Minuit.mnprofile: Now uses the same iterative robust fitting that
  Minuit.migrad uses. New keywords ncall, iterate, use_simplex allow
  to customize the fits. Strategy 0 is used first, and if that fails, the
  code switches to Strategy 2 in the iterations.
• Minuit.mncontour: Now uses the same iterative robust fitting that
  Minuit.mnprofile uses if experimental is True. New keywords ncall,
  iterate, use_simplex allow to customize the fits.
• A new tutorial illustrates the benefits of iterating and strategy on a pathological function. It is also shown that Scipy minimizers perform worse than Minuit when the function is pathological.