Add logos and clarify some docstrings

[cwhanse]

pvlib python pull request guidelines

Thank you for your contribution to pvlib python! You may delete all of these instructions except for the list below.

You may submit a pull request with your code at any stage of completion.

The following items must be addressed before the code can be merged. Please don't hesitate to ask for help if you're unsure of how to accomplish any of the items below:

• Closes issue Clarify clearsky.harwitz docstring #687
• 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.

[mikofski]

• check the markdown for the image on README, it needs a preceding exclamation mark if you want it to show the image, and not just link to the file.
• also is there a way to view the rendered docs? do you have a separate read the docs account for pull requests?
  thanks!

[mikofski]

need to add preceding exclamation mark for image to render, see GitHub Markdown guide

![pvlib-python](docs/sphinx/source/_images/pvlib_logo_horiz.png)

[cwhanse]

Thanks. Do you know how to specify a display size?

I can make a pre-release tag that will build on readthedocs. Otherwise I don't know how to view the change to index.rst

[wholmgren]

You can go into your readthedocs account and set up a new project that points to your fork. cwhanse-pvlib-python or something like that. Then you have to go into your new project's settings and turn on the build for your branch.

Alternatively, I can test locally if we're ready for that.

[cwhanse]

I set up a readthedocs for my fork, that is the long-term solution.

[cwhanse]

The index.rst file renders in the viewer here.

Ready for review.

[codecov]

Codecov Report

Merging #688 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #688   +/-   ##
=======================================
  Coverage   92.57%   92.57%           
=======================================
  Files          52       52           
  Lines        7485     7485           
=======================================
  Hits         6929     6929           
  Misses        556      556

Impacted Files	Coverage Δ
pvlib/irradiance.py	95.85% <ø> (ø)	⬆️
pvlib/clearsky.py	96.7% <ø> (ø)	⬆️

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[wholmgren]

Thanks. Looks good to me.

[mikofski]

Awesome, thanks! I'll go ahead and respond to NumFOCUS

[mikofski]

@cwhanse sorry to reopen this, but I think you should remove the height="260" from the <img> tag because it's rendering weird in my phone.

If I remove the height="260" then it renders fine:

[mikofski]

Note the Sphinx/Readthedocs html only uses width="600" and no height:

and renders fine on responsive devices:

[mikofski]

I did check when I clicked approve, and it seemed fine then, so not sure what happend :(

[cwhanse]

thanks for the attention to detail @mikofski !