Handle aoi>90 correctly in spectrum.spectrl2

[kandersolar]

• Closes spectrum.spectrl2 calculates negative irradiance for angle of incidence outside +/- 90° #1348
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Basically just copying over the two checks in haydavies.

[wholmgren]

Is it worthwhile to add a similar note to irradiance.haydavies? Something like "When supplying projection_ratio, consider constraining its values when zenith angle approaches 90 degrees. See code for details." Certainly not required here.