This is to filt Y = ax + bn + cs + dz + constant
xvec = list of valid x values, in the order used to construct Y
nvec = list of valid n values, in the order used to construct Y
svec = list of valid s values, in the order used to construct Y
zvec = list of valid z values, in the order used to construct Y
[Xg, Ng, Sg, Zg] = ndgrid(xvec, nvec, svec, zvec);
A = [Xg(:), Ng(:), Sg(:), Zg(:), ones(numel(Xg),1)];
b = Y(:);
xnsz1 = A\b;
a = xnsz1(1); b = xnsz1(2); c = xnsz1(3); d = xnsz1(4); constant = xnsz1(5);