add SingleAxisTracker support to ModelChain

[wholmgren]

This code makes it so that you can use a ModelChain with a SingleAxisTracker.

It's a bit verbose due to a couple of potentially bad design decisions in SingleAxisTracker (overriding PVSystem.get_irradiance and setting the output of singleaxis to nan for zenith > 90). I'd prefer to merge this for a 0.3.3 release and make the API breaking changes in 0.4.

The code works and passes a new test, but I want to test it more thoroughly in the next few days. I'll probably merge it on Tuesday or Wednesday unless someone objects.

Closes #169

To do:

• make a figure showing that it works
• add an assert_equals for the mc.tracking attribute
• add to whatsnew

[wholmgren]

Here's some code and a couple of figures demonstrating this feature plus the newly pulled scaling functions in #171.

module = modules['Canadian_Solar_CS5P_220M___2009_'].copy()
inverter = inverters['SMA_America__SC630CP_US_315V__CEC_2012_'].copy()

location = Location(32.2, -111, altitude=700)

system = SingleAxisTracker(module_parameters=module,
                           inverter_parameters=inverter,
                           series_modules=15,
                           parallel_modules=300)
mc = ModelChain(system, location)

times = pd.date_range(start='20160101 0000-0700', end='20160102 0000-0700', freq='1min')
ac = mc.run_model(times).ac

plt.figure()
ac.plot()
plt.ylim(0, 700000)
plt.ylabel('power (W)')
plt.title('Single axis tracker AC power calculated via a ModelChain, Jan 1')

times = pd.date_range(start='20160601 0000-0700', end='20160602 0000-0700', freq='1min')
ac = mc.run_model(times).ac

plt.figure()
ac.plot()
plt.ylim(0, 700000)
plt.ylabel('power (W)')
plt.title('Single axis tracker AC power calculated via a ModelChain, Jun 1')