Convert metadata in plasticc_gp()

[matwey]

Fixes: 211ecc1722f9 ("Add Plasticc dataset")

[codspeed-hq[bot]]

CodSpeed Performance Report

Merging #189 will degrade performances by 15.75%

_{Comparing fix/plasticgg (f9d8e3d) with master (57d275b)}

Summary

❌ 1 regressions ✅ 31 untouched benchmarks

:warning: Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	fix/plasticgg	Change
❌	test_benchmark_score[-1-1024-2]	70.9 ms	84.2 ms	-15.75%

[hombit]

@matwey could you please also fix the test that checks number of anomalies?

[matwey]

@matwey could you please also fix the test that checks number of anomalies?

Doen!