Instantiating a yaml camera takes a long time (10 seconds for imsim on my Mac). This can cause significant slow downs if there are repeat calls to yamlCamera.makeCamera with the same yaml file. This is particularly noticeable in tests. Since the camera returned by makeCamera should be immutable, it makes sense to add an lru_cache decorator around that call. This should also allow the local caching to be removed from obs_lsst.