Allow FixedView to be used to select other views

[HDembinski]

Minuit.fixed returns an object that mimics the numpy array interface and holds boolean values. The numpy expectation is that this array can be used as a mask for other arrays, but this currently does not work.

from iminuit import Minuit

m = Minuit(lambda x, y : x ** 2 + y ** 2, 1, 2)
m.fixed[0] = True

m.values[m.fixed] # returns [1, 2], not [1]
m.values[~m.fixed] # does not work, because the operator is not supported
m.values[m.fixed] = 0 # sets all values, not only those for m.fixed == 0

By the way, this works correctly:

err = np.array(m.errors)
err[m.fixed] = 0