New SEDCube and PolarizationCube classes to deal with new storage format
Improvements
Fixes for cubes that don’t have aperture information
Avoid copying the data into memory
Make messages debug instead of info
Create new MonochromaticFluxes class and adapt fitter to use this

We now always compute sizes on-the-fly: this is because otherwise it’s not straightforward to compute and store sizes for the monochromatic flues, and there is also the danger that the sizes could end up being out of sync with the fluxes, so easier - and fast enough - to compute on-the-fly.
Add support for new model cube in plotting
Removed stellar flux and polarization from SED class.

The stellar flux can be stored in an SED instance of its own, and the polarization can use a PolarizationSpectrum in future.
Re-implemented convolution for new model directory structure
Avoid duplication
More fixes
Implement ability to specify spectral order of SED cube (with views)
Try and fix issues with certain version of Numpy
Removed TODO [ci skip]
Take into account that model dir might change for different sources (if different output files were combined)
Removed TODO
Add docs about monochromatic fitting
Updated docstring
Added unit tests for new-style model directories and fixed issues.
Fix tests for Python 2