Is your feature request related to a problem? Please describe.
It would be nice to have the Marion 2008 [1] nonlinear adjustment to the PVWatts V5 DC power model (pvlib.pvsystem.pvwatts_dc) available in pvlib. This adjustment/improvement is already described on the Sandia PVPMC website [2] (with a typo from the original source included in the figure).

Describe the solution you'd like
I think a new function, e.g., pvlib.pvsystem.pvwatts_dc_marion, that is similar to pvwatts_dc with an additional input parameter of an irradiance correction factor, k, would be the best way to implement this.

An alternative would be a separate "adjustment" function that could be applied to the output of pvwatts_dc.

Additional context
Related to #2562 and this discussion, https://groups.google.com/g/pvlib-python/c/-3UuvBtWByY.

I had an offline exchange with @janinefreeman and @cpaulgilman to see if there was a specific reason that this adjustment was never implemented in PVWatts, and they could not find a record of any reason.

[1] B. Marion, "Comparison of Predictive Models for Photovoltaic Module Performance," IEEE PVSC 2008, https://docs.nrel.gov/docs/fy08osti/42511.pdf

[2] https://pvpmc.sandia.gov/modeling-guide/2-dc-module-iv/point-value-models/pvwatts/improvements-to-pvwatts/