Investigate impl From<Row> for concrete metadata types

[stuartlynn]

In moving from the struct based metadata to the polars DataFrame metadata, we have lost our strict typing on the metadata entries.

We could get this back by implmenting the impl From<Row> trait for the polars Row object.

Not sure if this is worth the work as we are increasingly working with subsets of the joined dataframe that don't neatly fit in to those old structures but leaving this issue here to explore in the future