Fix in MaxEnt_Batch: enumerate() returns (index, value) and not (value, index)

The current code works only if class names are integer starting with 0.
np.unique(...) returns something like [0,1,2,3], so enumerate(classes) = [(0,0), (1,1), (2,2), (3,3)] => c == c_i.
If other class id are used, it will not work.