spa license fixes

[wholmgren]

As discussed in #9, this PR:

• Removes NRELS's spa.c, spa.h, and spatester.c files.
• Updates setup.py so that it works with or without the spa files. I tested this, but see below.
• Updates test_solarposition.py to skip the spa tests if the files are missing. @bmu probably won't like the copy/paste test code, but I think that it can be fixed in 0.2.

(Now resolved) Major issue: NREL made a few minor changes to the SPA code that will require modifying the cython code. From the most recent spa.c file:

// Revised 10-JUL-2014 Andreas
// Change second in spa_data structure from an integer to double to allow fractional second
// Revised 08-SEP-2014 Andreas
// Corrected description of azm_rotation in header file
// Limited azimuth180 to range of 0 to 360 deg (instead of -180 to 180) for tech report consistency
// Changed all variables names from azimuth180 to azimuth_astro
// Renamed 2 "utility" function names for consistency

This PR currently works with the 01-APR-2013 version of the spa code. It should probably be updated to work with the latest version. I'm guessing that it would be a hassle to make it work with both versions.

This PR currently works with the 08-SEP-2014 version of the spa code. It does not work with the older code.

[wholmgren]

I think this is ready, but it should definitely be tested in full by somebody else.

[bmu]

I think pvlib/spa_c_files/spa_py.so should be removed from the repo and also be added to the ignore file (also because the SPA tests are passing before SPA is installed!)

[wholmgren]

Agreed, I'll fix this.

[wholmgren]

Hm, on second thought I already removed the file and line 10 of .gitignore is *.so. You may be seeing the file because of some git oddity when switching between branches.

$ git ls-files pvlib/spa_c_files/
pvlib/spa_c_files/README.md
pvlib/spa_c_files/SPA_NOTICE.md
pvlib/spa_c_files/__init__.py
pvlib/spa_c_files/cspa_py.pxd
pvlib/spa_c_files/setup.py
pvlib/spa_c_files/spa_py.c
pvlib/spa_c_files/spa_py.pyx

[bmu]

You are right, I see. Sorry that I was causing this confusion. The problem that the so will stay in the file structure exists, because it is ignored and will stay at its place, if you switch branches.

[bmu]

I checked out your branch locally and tested it. Everything seems to be ok (the only difference to your install instructions was the use of python setup.py install --inplace instead of pip install . (see my comment above). All tests were ok.

However I'm not sure about a Windows install, which could be a problem in gereneral because of the compilation of the c code (but this is not a problem introduced by this pull request).

[robwandrews]

I did testing on @wholmgren's branch. I think the confusion is coming from running the install from the local pvlib-python folder vs site-packages. When doing a pip install, the spa_c_files directory isn't created in site-packages when installing, so it isn't possible to go back and do a python setup.py install --inplace and have it apply globally. That will only work on the pvlib-python folder (the one that isn't in the site-packages directory).

So, I think the only way to propagate this into the site-packages directory properly is using pip, and so I'd reccomend updating the README.md with this:

To use the NREL SPA algorithm, a pip install from the web cannot be used. Instead: 

1. Download the pvlib repository from https://github.com/pvlib/pvlib-python.git
2. Download the SPA files from [NREL](http://www.nrel.gov/midc/spa/)
3. Copy the SPA files into ``pvlib-python/pvlib/spa_c_files`` 
3. From the ``pvlib-python`` directory, run ``pip uninstall pvlib`` followed by ``pip install . ``

[wholmgren]

I updated the readme with @Calama-Consulting's suggestion and rebased on the numerous master changes.