Resolved an issue where a visualization endpoint intermittently returned the wrong information

[GiaJordan]

Was able to repro erroneous behavior on prod, staging, and a local develop api, for visualize/tangled_tree/layers endpoint only.
Behavior was caused because over a session, the value for all_layers in TangledTree.get_tangled_tree_layers would be preserved between requests sent to the endpoint, with the new layer information just being appended to it. The endpoint then takes the first element of the all_layers list after it is returned, assuming that it is a list of length 1. What was actually happening is that the layers for each request were all being returned in this list and so when the first one was returned it could differ from what was requested in the most recent request.

[linglp]

@GiaJordan thank you for the PR. Could you deploy this to AWS before merging to develop?

[GiaJordan]

@linglp it's been deployed to the dev env. behavior is the same as on the local api

[linglp]

@mialy-defelice and @GiaJordan
I think having all_layers default as a list is likely to cause error in save_outputs (see the line here). The post here explains mutable default argument, and I think that's what caused the issue and confusion. Can we make some changes and make sure that all_layers don't get initialized as a list?

[GiaJordan]

@linglp that's a good point. I've modified the function according to the docs!

[linglp]

@GiaJordan @mialy-defelice I tested the PR and it looks good to me.