Add PVSystem pvsyst_celltemp methods

[CameronTStark]

• Closes issue Add pvsyst_celltemp method to PVSystem #633
• 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):

[CameronTStark]

I've added the methods to implement pvsyst_celltemp in PVSystem and ModelChain and PVSystem test that was straightforward as expected. I didn't get a clear sense of a test for ModelChain's implementation.

I took time to try to implement a test for the ModelChain method but found that the new "temp_model" possibilities introduced in pvsyst_celltemp don't work well with running ModelChain's defaults. I copied "test_modelchain.test_run_model_with_weather" and customized it for watching it run ModelChain.pvsyst_cell but ran into errors based on the provided pvsystem fixture. I made my own fixture which got complicated so I left it out of this PR and see if I could get some insight on this topic.

[cwhanse]

I changed #633 to address only PVSystem. If this PR can close the edited #633, we can take up the model chain work in another issue.

[mikofski]

Not sure what happens if default racking model is supplied when instantiating PVSystem because only "freestanding" and "insulated" are acceptable choices pvsyst_celltemp, not racking_model='open_rack_cell_glassback'

[mikofski]

Have you guys noticed that GitHub now alerts users when a commit is force pushed to a PR?

[CameronTStark]

The two failing checks seem to be from outside the changed code base from my PR. Is there anything else I need to do to get this accepted? Just making sure you're not waiting for me. Thx.

[cwhanse]

I'm ready to merge this. @wholmgren ?

[wholmgren]

Please see https://pvlib-python.readthedocs.io/en/latest/pvsystem.html for context on the requested API changes.

[CameronTStark]

Just FYI, the stickler-ci fail deals with code that I didn't contribute but I can change it if necessary.

[cwhanse]

@wholmgren Travis failure is a numpy/py27 conflict of some sort. py27-min build is OK. AppVeyor failure is test_read_midc_raw_data_from_nrel.

I don't know why the merged code from #638 shows up here as a change, it must be because it was merged into @CameronTStark's branch via a pull request from me, rather than from pvlib-python/master.

OK to merge?

[wholmgren]

LGTM. Thanks for the changes @CameronTStark.

Let's figure out the numpy issues in another issue. Could be a blocker for 0.6.1.