fix: raise cache memory warning threshold to 16 GiB

[anth-volk]

Fixes #549

Summary

Raise the first simulation cache memory warning from 8 GiB to 16 GiB while retaining the 32 GiB warning.

Changes

• Use 16 GiB and 32 GiB warning thresholds.
• Report GiB units consistently in the warning message and internal name.
• Cover values below 16 GiB, one warning per threshold, and warning-state reset after cache clearing.

Testing

• 9 passed in tests/test_cache.py
• Ruff format and lint passed.

This branch starts from PolicyEngine.py 6.2.0 on current main. It does not create a 5.2.x maintenance release or edit the package version manually; the release workflow will assign the next version after merge.