fix snlinverter and extraradiation doc string problems

[wholmgren]

@mikofski here's a partial fix for #242. I tried using a fancy table but it didn't make a difference. So, I moved the simple table out of the parameter description and into the notes section, which does improve the rendering:

http://wholmgren-pvlib-python-new.readthedocs.io/en/fixsnldoc/modules.html#pvlib.pvsystem.snlinverter

Maybe something about the RTD theme? In any case, I think this is an improvement and will merge soon unless there are objections or better ideas.

Here's the fancy table format that I tried.

    +--------+------------------------------------------------------------+
    | Column |  Description                                               |
    +========+============================================================+
    | Pac0   | AC-power output from inverter based on input power         |
    |        | and voltage (W)                                            |
    +--------+------------------------------------------------------------+
    | Pdc0   | DC-power input to inverter, typically assumed to be equal  |
    |        | to the PV array maximum power (W)                          |
    +--------+------------------------------------------------------------+
    | Vdc0   | DC-voltage level at which the AC-power rating is achieved  |
    |        | at the reference operating condition (V)                   |
    +--------+------------------------------------------------------------+
    | Ps0    | DC-power required to start the inversion process, or       |
    |        | self-consumption by inverter, strongly influences inverter |
    |        | efficiency at low power levels (W)                         |
    +--------+------------------------------------------------------------+
    | C0     | Parameter defining the curvature (parabolic) of the        |
    |        | relationship between ac-power and dc-power at the          |
    |        | reference operating condition, default value of zero gives |
    |        | a linear relationship (1/W)                                |
    +--------+------------------------------------------------------------+
    | C1     | Empirical coefficient allowing Pdco to vary linearly       |
    |        | with dc-voltage input, default value is zero (1/V)         |
    +--------+------------------------------------------------------------+
    | C2     | Empirical coefficient allowing Pso to vary linearly with   |
    |        | dc-voltage input, default value is zero (1/V)              |
    +--------+------------------------------------------------------------+
    | C3     | Empirical coefficient allowing Co to vary linearly with    |
    |        | dc-voltage input, default value is zero (1/V)              |
    +--------+------------------------------------------------------------+
    | Pnt    | AC-power consumed by inverter at night (night tare) to     |
    |        | maintain circuitry required to sense PV array voltage (W)  |
    +--------+------------------------------------------------------------+

[mikofski]

This is great! Sorry I haven't been contributing.

[dacoex]

@wholmgren seems like a RTD CSS issue. But haven checked with the native Sphinx theme.

[wholmgren]

Tests are failing because of unrelated forecast module problems, so going ahead with the merge.