add max_airmass=12 kwarg to disc

[wholmgren]

• Closes DISC Knc out of range #450
• I am familiar with the contributing guidelines.
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests (usually) must pass on the TravisCI and Appveyor testing services.
• Updates entries to docs/sphinx/source/api.rst for API changes.
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.
• Code quality and style is sufficient. Passes LGTM and SticklerCI checks.
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Pull request is nearly complete and ready for detailed review.

@cwhanse @adriesse ready for review.

[cwhanse]

I'm OK with this implementation. Exposing max_airmass in disc and diriint is fine.

[cwhanse]

This second sentence is not clear to me. Are you saying that the two functions produce different output with the same kwarg values, or that each function's output is different if the kwarg is set different from its default?

[wholmgren]

The outputs of each of these functions will be different than they were before if the user set the kwargs to more permissive values.

[wholmgren]

to be clear, this is only because of the change in disc and not because I've directly changed anything in these other functions.

[cwhanse]

Perhaps "Changing the value(s) of min_cos_zenith or max_zenith kwargs will change the output of :py:func:pvlib.irradiance.dirint and :py:func:pvlib.irradiance.dirindex`.

[wholmgren]

Thanks for the review @cwhanse. Would you be ok with not exposing the max_airmass kwarg in any of the functions? That is @adriesse's preference. If we keep the kwarg in disc I will leave it to others to do the work of exposing it in dirint and dirindex, if desired.

[cwhanse]

Would you be ok with not exposing the max_airmass kwarg in any of the functions?

Yes. My preference would be to expose it in all three functions, but I don't feel strongly about it.

[wholmgren]

I moved the airmass limit into _disc_kn so that it's also applied in gti_dirint. The test failure is unrelated. I think it's ready to merge.