compute MLE coefficients using iwls routine

    Methods: p189, Iteratively (Re)weighted Least Squares (IWLS),
    Fotheringham, A. S., Brunsdon, C., & Charlton, M. (2002).
    Geographically weighted regression: the analysis of spatially varying relationships.

def _compute_betas_gwr(y, x, wi):
    """
    compute MLE coefficients using iwls routine

    Methods: p189, Iteratively (Re)weighted Least Squares (IWLS),
    Fotheringham, A. S., Brunsdon, C., & Charlton, M. (2002).
    Geographically weighted regression: the analysis of spatially varying relationships.
    """
    xT = (x * wi).T
    xtx = np.dot(xT, x)
    xtx_inv_xt = linalg.solve(xtx, xT)
    betas = np.dot(xtx_inv_xt, y)
    return betas, xtx_inv_xt