Parset option for linear least square solver tolerances

Introduce parameters to set the tolerance of the linear least square solver. There are two parameters. The 'llstolerance' is the tolerance used in the final solver iteration (maxiter). The 'llsstarttolerance' is the value in the first iteration. For intermediate iterations, the tolerance is scaled so that it decreases with each iteration. If 'llsstarttolerance' is equal to 'llstolerance' or if it is not given, constant tolerance is used in the LLS solver.