Scripts for downloading reverse pourbaix data

[zccalka]

Adds a pre-computed reverse Pourbaix stability dataset and the scripts used to generate it. The dataset powers the reverse Pourbaix diagram tool on the MP website.

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • schema.py — defines the (pH, V) grid, parquet schema. Essentially the settings for the downloaded data.
  • pourbaix_cache.py — utilities for batch Pourbaix data generation: a SQLite-backed entry cache to avoid redundant API calls, a PourbaixDiagram subclass that skips redundant preprocessing when computing stability for many materials in the same chemical system, and a vectorized decomposition energy scan over the (pH, V) grid.
  • downloader.py — orchestrator script that queries the MP API, applies the above utilities, and writes the final parquet file consumed by the web app. TODO: Maybe this can be optimized further? Currently the download takes around 8 hours running from my hpc. The script also formats the parquet and json files. TODO: another parquet instead of the heatmap json file.
• I have run the tests locally and they passed.
  • Not applicable perhaps?
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR
  • Not applicable perhaps?

[zccalka]

@tsmathis for review