Prosail

There were issues with the pip package not including the spectral files. This should be fixed now.

Tested on Python 3, installs through pip and conda

**This is a pre-release version!!!**

I have added the PROSPECT-D model of Féret et al. (2017). The model is in essence the same PROSPECT as before, but with updated absorption spectra, and the addition of anthocyanins. PROSPECT has been re-written in Python (although the old FORTRAN version is still in the bindings), whereas SAIL is still a Python wrapper on the Fortran code. This module exposes both the PROSPECT model (both versions through the `prospect_d` method), as well as the possibility of coupling PROSPECT5 or D with SAIL (through the `run_prosail` method). I'm calling it a beta version because there's still documentation updates to be made, and some tests.

For more information on PROSPECT-D, see the following reference:
Féret, J-B., A. A. Gitelson, S. D. Noble, and S. Jacquemoud. "PROSPECT-D: Towards modeling leaf optical properties through a complete lifecycle." *Remote Sensing of Environment* 193 (2017): 204-215. [DOI](http://doi.org/10.1016/j.rse.2017.03.004)

A bugfix release. The code to call SAIL (i.e., not PROSPECT+SAIL) was buggy. I think it now works (but requires more testing than the one I could be bothered with).

Also, note that the calls to the canopy models have changed. They should be more streamlined, and they also allow you to get access to SDR, BHR, DHR and HDR.

The soil model can be overridden by passing a soil reflectance spectrum, but you can still use the old model (dry/wet soil) if you want to. This should make it easier to implement your model outside of the bindings (but remember that we still assume a Lambertian soil!)

This version is the same as the previous, but I wanted to have a DOI added to the repository. There. Done.