reading a pl.Array(pl.Float64) column back determines dtypes by first row

[TeunHuijben]

reading a pl.Array(pl.Float64) column back silently truncates fractional values to integers when the first rows happen to hold whole numbers

Reproduce by:

import polars as pl
import tracksdata as td

graph = td.graph.SQLGraph(drivername="sqlite", database=":memory:")
graph.add_node_attr_key("pos", pl.Array(pl.Float64, 2))
graph.bulk_add_nodes([{"t": 0, "pos": [50, 50]},     # whole numbers
                      {"t": 1, "pos": [1.5, 1.5]}])  # fractional

graph.node_attrs(attr_keys=["pos"])["pos"].to_list()
# -> [[50.0, 50.0], [1.0, 1.0]]     the 1.5 became 1.0