Improve lookup_linke_turbidity speed

[wholmgren]

• Closes issue lookup_linke_turbidity is slow #368
• Fully tested. Added and/or modified tests to ensure correct behavior for all reasonable inputs. Tests must pass on the TravisCI and Appveyor testing services.
• Code quality and style is sufficient. Passes git diff upstream/master -u -- "*.py" | flake8 --diff and/or landscape.io linting service.
• New code is fully documented. Includes sphinx/numpydoc compliant docstrings and comments in the code where necessary.
• Updates entries to docs/sphinx/source/api.rst for API changes. (marked methods as private, so no action)
• Adds description and name entries in the appropriate docs/sphinx/source/whatsnew file for all changes.

Improves the clearsky.lookup_linke_turbidity speed by 10-200x, depending on interpolation options and input data. See #368 for details.

@adriesse are you able to test this on your large dataset?

[cwhanse]

Looks good to me

[adriesse]

I'm afraid it would be difficult for me to test the new code. I have a workaround that I will leave in place until the next release. The only thing that matters though is the size of my time index, which is about 45M long with 1s freq.

[wholmgren]

Thanks for the review @cwhanse

@adriesse if it helps in the future...

you can install directly from github with pip install https://github.com/pvlib/pvlib-python/archive/master.zip

you can also replace the pvlib section of the url with another username, and the master section with the name of a branch. For example:
pip install https://github.com/wholmgren/pvlib-python/archive/ltspeed.zip