identify private methods, add _

[wholmgren]

This issue can be a place to list methods that should be identified as private methods. Private methods start with _, they do not show up in the documentation, and they should not be relied on by end-users.

* pvlib.pvsystem.golden_sect_DataFrame
* pvlib.pvsystem.Voc_optfcn
* pvlib.pvsystem.pwr_optfcn

Recommend a 0.2 tag.