fix: iminuit optimiser is never verbose (#724)

- minuit optimiser verbosity is now configurable instead of hard-coded off

Author: ntadej

src/pyhf/optimize/opt_minuit.py

@@ -18,7 +18,7 @@ def __init__(self, verbose=False, ncall=10000, errordef=1, steps=1000):
             verbose (`bool`): print verbose output during minimization
         
         """
-        self.verbose = 0
+        self.verbose = verbose
         self.ncall = ncall
         self.errordef = errordef
         self.steps = steps