irradiance.disc has unbounded clearness index calculation

[cwhanse]

In pvlib.irradiance.disc the clearness index kt is calculated as kt = ghi / I0h. Negative values of kt are removed but large values are not. In rare circumstances, ghi (e.g., from a clear sky model) greatly exceeds I0h yielding kt values upwards of 80. These values lead to a numpy overflow warning at

delta_kn = a + b + np.exp(c*am)`

because c is very large.

To demonstrate:
import pvlib
from pandas import Timestamp
ts = Timestamp('2016-07-19 06:11:00-0600', tz='US/Mountain', freq='T')
pvlib.irradiance.disc(ghi=1.0, zenith=89.99, datetime_or_doy=ts)

Expected behavior
irradiance.disc should use the new clearness_index function which sets an upper bound on kt

Versions:

• pvlib.__version__: 0.5.1