Dtype conversion issue

[vankesteren]

@qubixes you had excellent foresight

metasyn/metasyn/var.py

Lines 78 to 79 in c102905

	This method uses internal polars methods, so this might break at some
	point.

it broke. I fixed it for the categorical dtypes in #313, but we might want to do something a bit more stable / custom like have a direct lookup table from polars dtypes (e.g., pl.Categorical) to metasyn.