Function mean 

pub fn mean(data: &FdMatrix, dim: Dim) -> Vec<f64>

Compute the mean function for 1D or 2D functional data via a unified Dim dispatch.

The 2D path never diverged from the 1D one (both compute the pointwise mean over the flattened column-major grid), so both Dim arms forward to mean_1d. The dim argument makes caller intent explicit and provides a single future seam should a real 2D specialization ever be needed.

Arguments

• data - Functional data matrix (n x m)
• dim - Dimensionality selector (Dim::One or Dim::Two)