Earlier images in root-finding notebook

[IanHawke]

Linked to https://github.com/openjournals/jose-reviews/issues/58 .

This relates to https://github.com/albi3ro/M4/blob/master/Numerics/Roots_1D.ipynb .

The top of the notebook emphasises (correctly IMO) the importance of graphing the function. However, there's no pictorial examples until quite late: this makes the Multiplicity section not so easy to understand. In addition, there's no comment on what happens with multiple roots.

Suggest that the functions are introduced and plotted earlier, and that functions with multiple roots are also discussed. On the range you're using I'd suggest cos(2*pi*x)-0.4*x which has around 10 roots between -5 and 5.

[albi3ro]

I've added pictures and added a multiple roots example.

I'm still contemplating a rewrite of the section on "multiplicity" to remove the word "multiplicity" and just discuss "does it change signs?" and "does the derivative vanish?", but haven't quite put the pieces together yet.