Make it so that rather than defaulting to u.pixel in the metadata, u.pixel is used if the spatial_scale is missing - this leads to less confusing errors - e.g. 'spatial_scale is needed' rather than 'spatial_scale should be an angle' (the latter will be confusing if the user hasn't set the spatial_scale at all). Also improve consistency of variable naming in flux calculation compared to the rest of the analysis code.