Fix piezo model

[munrojm]

The latest version of Fast API does not support tuples with exact values (e.g. Tuple[int, int, int]) within pydantic models. This is due to OpenAPI specifications. It looks like this will get supported at some point, but this is a patch fix for now simply changing Tuple -> List in the model.

[codecov-commenter]

Codecov Report

Merging #295 (b3e728e) into main (b7a886c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #295   +/-   ##
=======================================
  Coverage   89.18%   89.18%           
=======================================
  Files          47       47           
  Lines        2477     2477           
=======================================
  Hits         2209     2209           
  Misses        268      268           

Impacted Files	Coverage Δ
emmet-core/emmet/core/polar.py	100.00% <100.00%> (ø)

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