Blending merged

[RobertGemmaJr]

Multiple models can now be passed to volume.js, the shader code will blend them together.

• Renames Aframe folder as aframe
• Defines model_structs uniform of type ModelStruct in fragment shader
• Removes intentisty, model_texture, and transfer_texture uniforms (now properties of ModelStruct)
• Removes create_model function - logic is moved to main
• Adds sample_model that samples each individual model
• Adds logic for Blending.None and Blending.Max blending types
• Reorganizes functions in volume.js
• Adds deepCopy utility function
• Updates README
• closes issue #68

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit df428a9):

https://volume-viewer--pr108-blending-merged-xrqh598c.web.app

_{(expires Mon, 13 Jun 2022 17:10:57 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

[kmilo9999]

@broarr @jashlu Quick review on the react code. Shader code is almost ready to go