fix demo notebook, bump version, improve typing. Release v0.3.1

[azmyrajab]

This PR provides minor fixes and cosmetic clean-ups:

1. improves the type annotations of python API functions to ExprOrStr indicating that user can pass shorthand column names if they wish (i.e. pl.col("y").least_squares.ols("x1", "x2") is a supported equivalent of pl.col("y").least_squares.ols(pl.col("x1"), pl.col("x2")))
2. fixes demo notebook @AdrienDart OLS - ComputeError: the plugin panicked #16. The demo notebook should now be more comprehensive and explains "SolveMethod" and "NullPolicy" usage
3. forces a "rechunk" on rust series prior to conversion to ndarray because that step ensures contiguous memory and zero-copy to ndarray
4. bumps version to v0.3.1