narwhalify MeanResponseTransformer

[limlam96]

make MeanResponseTransformer polars compatible while maintaining backwards compatibility with pandas.

Why?

Making the project polars compatible for speed benefits. Narwhals allows us to do this piece by piece and maintain compatibility between components as it makes it possible to pass pandas dataframes to polars logic. This involves converting between pandas/polars dataframes.

Note, over time this transformer has become a bit sprawling - as part of this ticket, should evaluate whether it is worth splitting out into child/parent classes (e.g. MeanResponseTransformer and MeanPriorResponseTransformer). If this route is taken, I would spin out into a separate initial issue and PR.

How?

convert logic in MeanResponseTransformer to function with polars dataframes and add narwhalify decorators to methods to ensure compatibility with pandas dataframes.

Update testing where required.

An example narwhalification PR can be found #356