Improve clarity of code in pvsyst_celltemp (names and doc)

[adriesse]

• Improves on the implementation of issue ENH: Implement PVSyst Cell Temperature Model #552
• 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.

Brief description of the problem and proposed solution (if not already fully described in the issue linked to above):

I have tried to improve clarity of code in pvsyst_celltemp through changes to variables names and docstring, and also changed the name of one function parameter.

[cwhanse]

see comments on code lines.

[adriesse]

Here are some additional thoughts about things I did not include in the PR:

• In the function argument list, I believe windspeed would be better placed after temperature, and given a default value--probably zero.
• If alpha is to be exposed, then it could/should be dealt with in the same way as the loss factors rather than as an independent function argument. If this were done, then the name loss_factors would definitely no longer be appropriate, and I would suggest something like model_parms instead.
• For my own work I still like dictionaries for presets, so my functions don't need to know about them. So I would use:
  • pvsyst_celltemp (poa, wind, temp, **freestanding) or
  • pvsyst_celltemp (poa, wind, temp, **model_parms['freestanding'])

I do have some new work on temperature models which will raise some additional interesting code integration questions, but that's still a few months away at best.

[wholmgren]

In the function argument list, I believe windspeed would be better placed after temperature, and given a default value--probably zero.

For my own work I still like dictionaries for presets, so my functions don't need to know about them. So I would use: pvsyst_celltemp (poa, wind, temp, **freestanding) or pvsyst_celltemp (poa, wind, temp, **model_parms['freestanding'])

I agree with these proposals. They make the low level functions more flexible and more explicit. The doc strings can point users to the presets in the module-level dictionary. The PVSystem and ModelChain abstractions can still make the function a bit easier to use for common use cases.

We'd need a deprecation cycle to make the same changes to sapm_celltemp. I don't have a preference about if that happens for 0.6.1 or not.

[adriesse]

Further changes anyone?

[wholmgren]

Ok to merge if CI passes.

[cwhanse]

I should have checked the argument order for the method earlier, sorry I did not catch that. I can't tell why the test is failing, @wholmgren any ideas?

[wholmgren]

@cwhanse I think the failure was due to the mismatched arguments, but it looks ok now.

I just noticed that wind_speed is required in the PVSystem method but optional in the function. Is that the intended behavior?

The codecov test is failing for reasons unrelated to this PR and can be ignored.

[cwhanse]

one more minor fix

[cwhanse]

@adriesse thanks Anton!

[adriesse]

You're welcome!