SAS Model Organization

Models have certain features in common.

Every model has a scale and a background.

Talk about orientation, with diagrams for orientation so that we don't need a link on every model page?

Talk about polydispersity.

Talk about magnetism, converting the magnetism help file to inline text here, with links so that models can point back to it.

Need to talk about structure factors even though we don't have any implemented yet.