add optional ivcurve calculation to singlediode

[wholmgren]

This PR adds an IV curve calculation feature to pvsystem.singlediode.

As discussed in #83, the singlediode API remains unchanged for timeseries analysis: series in yields dataframe out. Scalar and array input still yield dict output (well, now an OrderedDict).

Setting the new ivcurve_pnts keyword argument to a non-zero number will always yield an OrderedDict. The dict will also include the keys i and v. The values will be arrays with dimension one greater than the input data (1d arrays for scalar input, 2d arrays for array/series input).

Hopefully that makes sense. The idea is that it "just works".

Example usage below.

Closes #83.

In [197]: times = pd.DatetimeIndex(start='2015-06-01', end='2015-06-02', freq='3H', tz='America/Phoenix')

In [198]: tus = pvlib.location.Location(32, -110)

In [199]: cs = tus.get_clearsky(times)

In [200]: cec_module_params = pvsystem.retrieve_sam('cecmod')['Example_Module']

In [201]:     IL, I0, Rs, Rsh, nNsVth = pvsystem.calcparams_desoto(
     ...:                                          cs['ghi'],
     ...:                                          temp_cell=25,
     ...:                                          alpha_isc=cec_module_params['alpha_sc'],
     ...:                                          module_parameters=cec_module_params,
     ...:                                          EgRef=1.121,
     ...:                                          dEgdT=-0.0002677)

In [202]: sd = pvsystem.singlediode(IL, I0, Rs, Rsh, nNsVth, ivcurve_pnts=100)

In [203]: for i, v, poa in zip(sd['i'], sd['v'], cs['ghi']):
     ...:     plt.plot(v, i, label='{:.0f}'.format(poa))
     ...:

In [204]: plt.ylabel('current (A)'); plt.xlabel('voltage (V)')
Out[204]: <matplotlib.text.Text at 0x1068fdd68>

In [205]: plt.ylim(0, None); plt.xlim(0, None)
Out[205]: (0, 12.0)

In [206]: plt.legend()
Out[206]: <matplotlib.legend.Legend at 0x1068fcc88>

I would add the 5 IV curve points to the plot, but this PR does not include the fix in #222.