FEAT - Implement path fitting for SLOPE

[jolars]

Description of the feature

Implement fitting the full SLOPE path in the same way as is already done for the lasso, for instance.

Considered alternatives

Additional context

[jolars]

Ah, I see why this might be slightly more involved than I though. The path fitting is tied to AndersonCD, which doesn't work for SLOPE. So one would have to look into implementing a path function for FISTA first, perhaps.