graphic output: initial estimations xlim, ylim

2 new graphical output params have been added:

xlim {(nan, nan)}, ylim {(nan, nan)} - initial estimation for graphical output borders

you can use for example p.xlim = (nan, 10) or p.ylim = [-8, 15] or p.xlim=[inf, 15], only real finite values will be taken into account

Of course, you can use p = NLP(..., xlim=[8,15], ylim=asfarray(nan, 15), ...) as well

for constrained problems ylim affects only 1st subplot.