fix or silence expected numpy RuntimeWarnings

[wholmgren]

A handful of pvlib functions generate numpy RuntimeWarnings for typical inputs. We should tweak the pvlib code or wrap specific lines with np.errstate context managers. Pandas used to silence numpy RuntimeWarnings, but they wisely changed that in 0.18 or 0.19.

These functions and linked lines need work:

• perez
• physicaliam
• sapm
• i_from_v

I'm probably missing a few, so I'll add to this as we discover more.

[wholmgren]

It seems that many, if not all, of these runtimewarnings are actually due to some combination of compiler options and numpy bugs. So, nothing for pvlib to do here.