Allow for function decrease in LM optimizer.

A new parameter was introduce to specify the acceptable function decrease in an iteration. Acceptable means that the new parameters are kept for continuing the iterations. This behaviour existed already before, but a low threshold of 1e-6 did not really activate it. Now the threshold was raised to 1.0 and this help ctlike to converge quicker. Methods accept_dec() have been implemented to get and set the acceptable threshold.