Work with model atmospheres

Currently, the on-the-fly synthesis only works at model-atmosphere gridpoints. Do the following:

[x] Write a new module apogee.modelatm that contains operations on the model atmospheres;
[x] Need apogee.modelatm.kurucz for the model atmospheres that we're using now;
[x] This should allow one to easily display the properties of the model atmosphere;
[x] Should probably also calculate the Rossland mean optical depth and use that as the independent variable in plots;
[x] Then write an interpolator of model atmospheres; probably best to follow the established route:
- [x] Find closest grid points (remember, we have to potentially do this in 5D Teff,logg,metals,O/Fe,C/Fe);
- [x] Interpolate each model atmosphere on the grid onto a common optical depth scale;
- [x] Then interpolate each layer;
- [x] Need to write the output to a file, probably best in the original ATLAS format that the grid is on
- [x] Need to take care to properly write down the ABUNDANCE CHANGES
- [x] Anything else?
[x] Then allow use of these in the MOOG spectral synthesis.

jobovy / apogee