Changed secondary loss calculation (solves #43)

[arognoni]

In the original code losses were multiplied (instead of being added) and frequencies were added (instead of being multiplied). Now the code works as expected:

model1 = pyfair.FairModel(name="Insider Threat", n_simulations=10)
model1.input_multi_data('Secondary Loss', {
    'Reputational': {
        'Secondary Loss Event Frequency': {'constant': 1},
        'Secondary Loss Event Magnitude': {'constant': 10},
    },
    'Legal': {
        'Secondary Loss Event Frequency': {'constant': 1},
        'Secondary Loss Event Magnitude': {'constant': 10},
    }
})

model1._model_table["Secondary Loss"] returns:

0    20.0
1    20.0
2    20.0
3    20.0
4    20.0
5    20.0
6    20.0
7    20.0
8    20.0
9    20.0

model2 = pyfair.FairModel(name="Insider Threat", n_simulations=10)
model2.input_multi_data('Secondary Loss', {
    'Reputational': {
        'Secondary Loss Event Frequency': {'constant': 0},
        'Secondary Loss Event Magnitude': {'constant': 10},
    },
    'Legal': {
        'Secondary Loss Event Frequency': {'constant': 0},
        'Secondary Loss Event Magnitude': {'constant': 10},
    }
})

model2._model_table["Secondary Loss"] is equal to:

0    0.0
1    0.0
2    0.0
3    0.0
4    0.0
5    0.0
6    0.0
7    0.0
8    0.0
9    0.0

model3 = pyfair.FairModel(name="Insider Threat", n_simulations=10)
model3.input_multi_data('Secondary Loss', {
    'Reputational': {
        'Secondary Loss Event Frequency': {'constant': 1},
        'Secondary Loss Event Magnitude': {'constant': 10},
    },
    'Legal': {
        'Secondary Loss Event Frequency': {'constant': 1},
        'Secondary Loss Event Magnitude': {'constant': 10},
    },
    'Response': {
        'Secondary Loss Event Frequency': {'constant': 1},
        'Secondary Loss Event Magnitude': {'constant': 10},
    }
})

model3 does not return errors anymore.