158 magnetism in refl1d

[andped10]

Another step in the process of getting the ability to handle magnetism into EasyReflectometry.

First version of tutorial demonstrating how the magnetism of layers can be set.

I know the way to set the magnetism of a layer should be more elegant, but consider this a step towards that.

Issues that should be handled after this PR:
#168
#167

[jfkcooper]

A couple of suggested rewordings and comments on the notebook, but otherwise looking good

[jfkcooper]

Thanks!