Creates a notebook showing how Synfit works

This is still a work in progress but it is a nice showcase.
Define some variables as global.

I have two examples and they some variables in common.
Adds support to no log-scale

The two paameter chi-square plot was always plotted in log-scale. I
added it as optional.

I also changed the way it calculates the log.
Instead of calculating the log of chi-square, I added it as an argument
to the pyplot.imshow command and let matplotlib do the calculation. I
checked with one plot and the plots looked alike.
Explicit the number of parameters when plotting

In the function plot_chisquare, I created a variable that stores the
number of fitted parameters. I hope this will make it more clear.

I also removed a try..except sentence that caught the error if there was
not just 2 parameters. It was not needed because I make sure that there
are two with a if sentence some lines before.
Adds support to fixed values of abundance.

If the suer want abundances with fixed value, he/she can pass it as a
parameter on the kwargs.