More robust fix for nested wavelength arrays

[stephtdouglas]

When the wavelength table is being created from the model database (see make_model.retrieve_model on the paige branch has this) it nests the wavelength arrays in an odd way. When getting the correct wavelength array, you have to go a couple levels down (more than you should) to get to the right array.

Also, the code should be robust to getting either a single wavelength array for the entire model grid or a separate wavelength array for every model.