Add in component sum equations for GHI, DHI, and DNI into the pvlib.irradiance module.

[kperrynrel]

Right now, we have code for calculating GHI, DHI, and DNI via the component sum equations in pvlib/pvanalytics, and doing nighttime corrections (pvlib/pvanalytics#163). @cwhanse and @kanderso-nrel suggested we port over the 3 component sum equations to the pvlib.irradiance module, and call them directly in pvlib/pvanalytics before applying a nighttime filter. This would require taking the guts out of pvlib.modelchain.ModelChain._complete_irradiance and creating a new function in pvlib.irradiance.

[cwhanse]

Said differently: we're proposing to promote the ModelChain method _complete_irradiance to a public function in pvlib.irradiance.