Broken link in pvsystem.retrieve_sam

[cwhanse]

The link to documentation of CEC parameter sets is broken.

Expected behavior
Link to a current webpage? A bit of searching hasn't turned up a target for this link.

I don't think pvlib should be responsible to maintain a description of those databases. I favor just removing the dead link.

[cwhanse]

@janinefreeman

[kandersolar]

archive.org says it's just the PV publications page: https://web.archive.org/web/20200618135912/https://sam.nrel.gov/photovoltaic/pv-sub-page-2.html

I agree that the link is not really helpful for these databases, but for completeness, the new url is: https://sam.nrel.gov/photovoltaic/pv-publications.html

[janinefreeman]

Let me know if you need any other links!