add a function that multiplies voltages and currents for arrays

[wholmgren]

I'd like to consider adding a function to pvsystem.py that would multiply the voltages and currents returned by pvsystem.sapm and pvsystem.singlediode. Of course, the motivation for this is to make it easier to calculate array parameters.

def multiply_voltage_current(data, voltage_multiplier=1, current_multiplier=1): 
    # as written, only works with a DataFrame
    # could make it work with a dict, but it would be more verbose
    data = data.copy()
    voltages = ['v_mp', 'v_oc']
    currents = ['i_mp' ,'i_x', 'i_xx', 'i_sc']
    data[voltages] *= voltage_multiplier
    data[currents] *= current_multiplier
    data['p_mp'] *= voltage_multiplier * current_multiplier

    return data


# within the PVSystem class...

    def multiply_voltage_current(self, data):

        return multiply_voltage_current(
            data, voltage_multiplier=self.series_modules,
            current_multiplier=self.parallel_modules)

Maybe the function can also be reused for some system losses.

[wholmgren]

Closed by #171.