* Add out_indices to backbones, deprecate out_features
* Update - can specify both out_features and out_indices but not both
* Add backbone mixin tests
* Test tidy up
* Add test_backbone for convnext
* Remove redefinition of method
* Update for Dinat and Nat backbones
* Update tests
* Smarter indexing
* Add checks on config creation for backbone
* PR comments
