move all the functions to lib.c and lib.h; add this two librearies to makefile;
make plot.py more easy to use for passing to one mode to the other (show or
save figure); create fit_plot and create the figure 5-fit.pdf.
A for-cycle has been implemented in order to produce a bunch of
results with different numbers of function-calls which can now
be plotted with plot.py, which has been added to the folder.
