solarposition integration round 3

Addresses some issues brought up in #27.

I added pythonic-PVLIB's version of ``pvl_ephemeris,py`` to ``solarposition.py``. I used this version because it has better support for pytz, fixes some integer division errors, and returns a single DataFrame. I also added a logging warning that points users towards the other solar position functions.

I made a few changes to ``TS_2_Irradiance_functions.ipynb`` but stopped when I ran into trouble with the airmass calculator. That module will have to be addressed separately. I also got confused about why there are notebooks in the doc folder and in the tutorials folder and which ones @Calama-Consulting wanted me to update. Again, for another issue.

To get the notebook working without explicit submodule imports, I added them to ``__init__.py`` (as discussed in #10).