Function compute_field_laplacian 

pub fn compute_field_laplacian(
    field: &[f32],
    mask: &[u8],
    nx: usize,
    ny: usize,
    nz: usize,
    vsx: f32,
    vsy: f32,
    vsz: f32,
) -> Vec<f32>

Laplacian of an unwrapped field (any units), zero outside mask.

Ordinary 7-point discrete Laplacian with replicated boundaries. This is what TgvInput::FieldPpm uses: unlike compute_phase_laplacian it is linear, so it is exact for fields whose neighbour differences are not small compared with 1.