Centers beta so that weighted sum over s equals 0 for each t: sum_s w(s) * beta(s, t) = 0 for all t. This matches pffr's identifiability constraint for ff terms. Uses Simpson weights to be consistent with pffr's default integration.
center_ff_beta(beta_st, xind)Centered beta matrix with same dimensions.
Numeric matrix (n_xind x n_yind) coefficient surface.
Numeric vector of x evaluation points (for integration weights).