Better method for distinguishing 1d and 2d opacity files

[isaacgsmith]

Some opacity cross-section files only show cross-section vs. wavelength, while others do cross-section vs. wavelength *and temperature. The current solution is that we check for a blank entry in the top left corner of the csv to see if it is a 2d array of cross-sections or not (as in this case the left column would be wavelength and the top row would be temperature, and thus the top left would be blank). There is probably a less-hacky way of doing this -- we should look into that.

The solution to this may also be tied to the solution of #42. Also, this should be kept in mind when generating new opacity files (#41).