Python library to calculate the index of refraction of air in the near infrared.

This library offers three models to calculate the index of refraction. The first is a simple equation from Henry Roe's paper (http://arxiv.org/pdf/astro-ph/0201273v1.pdf). Its implementation is named nRoe. The next is a multivariable Taylor expansion from Richard Mather (http://arxiv.org/pdf/physics/0610256v2.pdf). Its implementation is named nMather. The third one is a Sellmeier-equation based model from  Voronin & Zheltikov (2017) (https://www.nature.com/articles/srep46111). This model has not been tested for correctness currently.

While I did not find an explicit range of valid wavelengths for Roe's equation, the code for Mather's equation is only valid between 1.3 and 2.5 microns and 2.8 through 4.2 microns. There are longer wavelengths in his paper, but have not been implmeneted here. 

Also included is a python script that plots a comparison of the first two equtions. They agree up to a precision of 1E-7 in the near infrared (1.3 - 2.5 microns).