Support multichain models with associated PAEs

PAE matrices for models including interacting protein chains are not supported in 2.11.3:
1. It should be easy to import and view 3d models and PAEs for interacting proteins. (possible but very fiddly in 2.11.3, and only one chain per model actually gets the PAE matrix added to it).
2. Each sequence should show the correctly mapped PAE - optionally with horizontal lines to delineate PAE scores for different chains. (Not possible in 2.11.3)
3. Tree calculation, clustering & grouping by PAE should happen once on the PAE, with each mapped chain inheriting groups and clusters. (see JAL-4124)