Add Fuentes temperature model

[wholmgren]

Is your feature request related to a problem? Please describe.
PVWatts uses a temperature model, Fuentes, that's not implemented in pvlib python.

Describe the solution you'd like
Add the Fuentes model described in this pdf: 850330.pdf

Describe alternatives you've considered
Continue to use sapm.

Additional context
Originally discussed in #1022 (comment)

[kandersolar]

I have a python implementation of the Fuentes model I can contribute. However because the model includes a thermal inertia effect, each temperature calculation depends on the previous value in a nontrivial way and I haven't found a way to vectorize it with numpy, so unfortunately it's a little slow. Calculating cell temperature for an 8760 takes 4-5 seconds on my computer. Is slow code better than no code? If so, I will open a PR.

The code is pretty similar to the original FORTRAN77 code in the reference's Appendix A... it would be a good candidate for rewriting in cython if pvlib goes that way.

[wholmgren]

Numba is also a good option.