Add basic Turbospectrum support

[andycasey]

This pull request:

• Updates the MARCS photospheres that are downloaded with SMHr to include radiative pressure and model radius
• Incorporates Turbospectrum support for synthesising spectra
• Incorporates Turbospectrum support for calculating equivalent widths from abundances
• Incorporates Turbospectrum support for calculating abundances from equivalent widths
• Updates LineList class to write line lists in Turbospectrum format
• Updates Photosphere class to export MARCS photospheres in Turbospectrum format

Current caveats/future improvements:

• Turbospectrum must be installed separately and bsyn_lu (and other executables) must be on $PATH
• You must have a $TURBODATA environment variable that points to the Turbospectrum DATA/ folder
• Since some of the SMHr default line lists don't include all the transition attributes required for Turbospectrum, we are just fudging them
• Molecules are not currently considered when running Turbospectrum

[alexji]

Can you hook this into the session/GUI? Also would be good to have some instructions (e.g. on readme) for downloading new model atmospheres, and to fix Travis so some tests can run.

We should also merge this into v0.2, which should become the master branch in the next few months. (I can do this.) There are more tests in v0.2 than master too, and I'll have them run with both TS and MOOG.

BTW my first test case will be to do manual spectroscopic stellar parameters for the Frebel+13 eqws, which should be a good science test as well.

[andycasey]

I will hook this into the session/GUI this weekend.