Feature/support gradient arrays in firebase

[rugeli]

Problem

What is the problem this work solves, including
closes #273

Solution

When gradient is a list of gradient names, e.g "gradient": ["X_gradient","Y_gradient"], we need to add additional steps to our current upload+download system.

• when uploading: added handling for gradient lists to record each reference of gradient in the list
• for duplicate check: resolve each gradient data in the list correctly and consistently
• when downloading: prepared data to recognize gradient lists and fetch each gradient data correctly

Note: After adding logic to detect, unpack and resolve combined gradients in the corresponding steps of the process, our system can now upload and download the current test recipes in all contexts. I did notice an odd behavior (not breaking anything) with the recipe loader automatically adding a default representation to each object when downloading recipes from firebase, will write another issue to address this.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Steps to Verify:

1. Ensure your database doesn't contain any test_combined_gradient.json related data. Delete them if they exist
2. Upload recipe to firebase: run upload -r cellpack/tests/recipes/v2/test_combined_gradient.json
3. Pack the firebase recipe: run pack firebase:recipes/test_combined_gradient_v_1.0.0
4. If followed step 1, you should be able to upload and download without issues. (but please let me know if you do encounter any)

[github-actions]

Packing analysis report

Analysis for packing results located at cellpack/tests/outputs/test_spheres/spheresSST

Ingredient name	Encapsulating radius	Average number packed
ext_A	25	236.0

Packing image

Distance analysis

Expected minimum distance: 50.00
Actual minimum distance: 50.01

Ingredient key	Pairwise distance distribution
ext_A

[rugeli]

This section of code shares some similarities with def get_reference_in_obj() in CompositionDoc class, but they perform distinct tasks -- this one unpacks gradients for downloading, the latter resolves db references for duplicate checking while uploading. For clarity and easier maintenance, I'm going to leave them as standalone sections.

[mogres]

Tested out with the combined gradient recipe and it works for me!