Synfit: Plot ticks only on fitting values

The ticks of the chi-square distribution plots will be only the fitted
values.

I hope that this helps clarify the plot.
Synfit: rotate tick labels if chemical element

Since values of chemical elements used usually have two decimal places,
the label could get crowded. Then, I am rotating a little avoid it.
Synfit: Adds grid to chi-square plot

This will help the user find the values fitted.
Update example notebook

I rerun to show the last updates on Synfit chi-square distribution plot.
Merge pull request #27 from gabraganca/feature-chisquare-plot

Improves chi-square distribution plot