fix: handle missing scatter_data in plot_centiles_advanced

[divye-joshi]

Fix: handle missing scatter_data in plot_centiles_advanced

Closes #373

Problem

plot_centiles_advanced crashes when called without scatter_data in two scenarios:

1. Saving plots: plotname is only defined inside if scatter_data:, but the save_dir logic at the end uses it unconditionally → NameError
2. Multi-covariate models: The midpoint computation for non-plotted covariates accesses scatter_data.X unconditionally → AttributeError

Changes

Minimal two-line fix in plotter.py:

1. Default plotname: Added
   plotname = f"centiles_{response_var}"
   before the if scatter_data: block so it always has a value when saving.

2. Covariate fallback: When scatter_data is None, the non-plotted covariate value now falls back to the midpoint of the model's stored covariate range instead of trying to compute the mean from scatter data.

Verification

After the fix, the following calls work without error:

# Save centile-only plot
plot_centiles_advanced(model, save_dir="./plots")

# Multi-covariate model without scatter data  
plot_centiles_advanced(multi_cov_model)

[contsili]

the same errors happen with the plot_centiles too. I will fix them in this branch and merge everything tomorrow. Thank you @divye-joshi!

[divye-joshi]

@contsili good point! thanks! I encountered this when using the notebooks where plot_centiles_advanced was used so i overlooked plot_centiles.