pvsyst_system_losses

[btaute]

Is your feature request related to a problem? Please describe.
An implementation of PVSyst system losses would help new users.

Describe the solution you'd like
I'd like an implementation of dc_losses_pvsyst and ac_losses_pvyst, similar to the code below.

Describe alternatives you've considered
PVWatts_losses implements losses differently.

Additional context
I'm happy to lead the Pull Request on this. Additional pull requests would be to implement each of the losses, which are simply getting compiled here. Most losses would be a Series of fractions, but some of them (auxiliary_power and external_transformer_losses) would be in watts.

def dc_losses_pvsyst(dc_power, dc_cabling, module_quality, lid,
                     module_mismatch, strings_voltage_mismatch,
                     ageing):
    loss_factor = combine_loss_factors(dc_power.index, dc_cabling,
                                       module_quality, lid, module_mismatch,
                                       strings_voltage_mismatch, ageing)
    dc_losses = dc_power * loss_factor
    return dc_losses


def ac_losses_pvsyst(ac_power, ac_cabling, unavailability, auxiliary_power,
                     external_transformer_losses):
    loss_factor = combine_loss_factors(ac_power.index, ac_cabling,
                                       unavailability)
    ac_losses = ac_power * loss_factor + auxiliary_power
                                       + external_transformer_losses
    return ac_losses