Change interpolation method of plot

Instead of doing a cubic interpolation, it now does a linear
interpolation. This is more representative of the data measured.
With the cubic interpolation, I could get a value from the plot
different from the fitted.
Fix plotting of chisquare when abund was parameter

When one of the parameter was a chemical element, the plot returned and
error. It was raising the error that it had 2 or more elements, but it
was not. It was catching a wrong error. The problem was with converting
a string from the value of abundance.

I think that it is still catching the error in a wrong way.
Change how to catch error.

Instead of using a try..except, I changed to the if..elif..else:raise.
If I think on a better way I will change it back.
Replace code for a more general

This is regarding the plot_chisquare function

I have written this code to be replace the one when finding the best
fit, but the result was the same as it is now (when using a linear
interpolation). Since, in the base, this code was derived from the
previous one on the plot_chisquare function, I decided to replace it.

I hope I did not make it more difficult to understand and then going
against the Zen of Python.