Consider making i_x and i_xx SAPM parameters optional

[kandersolar]

Is your feature request related to a problem? Please describe.
The SAPM DC model produces what it calls the $I_x$ and $I_{xx}$ points in addition to the usual SC/OC/MP points. These require additional parameters which may be hard to determine, as these points are not included in module characterization methodologies like IEC 61853-1. They are also not very useful; applications requiring such intermediate points are probably better off using a SDM.

Describe the solution you'd like
Make the $I_x$ and $I_{xx}$ parameters (IXO, IXXO, C4, C5, C6, C7) optional in pvlib.pvsystem.sapm, and any higher-level wrappers like ModelChain.

Describe alternatives you've considered
N/A

Additional context
#2393 (comment)