Third option to remove low solar elevation artifact in clearsky.ineichen

[martinherrerias]

See #435 and #459 for context.

One way to remove the artifacts while introducing less bias is to clip the airmass_absolute values to the point where the exponential correction becomes problematic. Something like:

airmass_critical = (cg2*(fh1+fh2*(tl-1))/0.018).^1.25
airmass_clipped = np.fmin(airmass_absolute, airmass_critical)
ghi = (np.exp(-cg2*airmass_clipped*(fh1 + fh2*(tl - 1)) + 0.01*airmass_clipped**1.8))
...
ghi = cg1 * dni_extra * cos_zenith * tl / tl * np.fmax(ghi, 0)

airmass_critical is a minimum for the combined exponential terms, which shoot up afterwards. Loosely:
airmass_critical = argmin(f(x)), for f = exp(-cg2*x*(fh1 + fh2*(tl - 1)) + 0.01*x^1.8))

This threshold starts at AM = 2.6 (for turbidity = 1 and altitude = 0), equivalent to ~22.5° solar elevation, and goes up to O(10) for higher turbidities and altitudes. For most cases, it corresponds to solar elevation angles below the 4° cutting threshold of the CIE 1994 quality control guidelines that Ineichen & Perez seem to have used, i.e. they probably didn't see beyond it.

Clipping at the minimum makes the adjustment smooth (C1), and leaves the results of the model untouched for most operating conditions.

Originally posted by @martinherrerias in #435 (comment)

[martinherrerias]

@mikofski, there is no reference for this fix, and I also had no plans of publishing it. I take comfort in the fact that the proposed correction falls between the two (already published) versions of the model, but @adriesse is right to point out that actually none of the models is validated for very low solar elevations.