get rid of LinearMap's find_copy method

As with the now removed `get_copy` method, this is the same as just doing a copy in the caller and doesn't offer an advantage in either simplicity or performance (it also doesn't work with `Clone`). It duplicated the implementation of the `find` too.

This was still necessary until the recent changes to the borrow checker that allow reborrowing `&mut` as `&`. The update to `workcache.rs` shows an example of the benefit from that change (`find_copy` or copying the entire map was required before).