Update to hspline and lowess filters.

[talensgj]

Hello,

I've made some modifications to the hspline and lowess filters, specifically:

hspline

• Add StandardScaler to improve stability.
• Call HuberRegressor with max_iter=1000 to improve stability/convergence.
• Expose HuberRegressor epsilon through cval.

lowess

• Change the way the left and right indices of the window are computed to use np.searchsorted, fixing a bug.
• Simplify calc_x_weights, by changing y_weights intilisation.
• Expose the y_weight cut-off through cval.

I hope you will find these changes make sense and merge them into wotan.