Add calculate dhi option to PVSystem class

[javlor]

No description provided.

[iantsednv]

@javlor, let's add some unit tests for this following the pattern used for enable_spectral_modeling for example:

def test_calculate_dhi_off_by_default(self, plant):
    payload = json.loads(construct_plant(plant))
    opts = payload["energyCalculationOptions"]
    assert opts.get("calculateDHI", False) is False

def test_calculate_dhi_on_when_enabled(self, plant):
    plant.calculate_dhi_weather = True
    payload = json.loads(construct_plant(plant))
    assert payload["energyCalculationOptions"]["calculateDHI"] is True

[iantsednv]

A few comments