Section on multi-echo combination methods and related equations

[jsheunis]

I have some content on multi-echo combination methods that could perhaps be useful in the book. The first file is a very basic explanation of summation, weighting and averaging; the second is a summary (+equations) of the main combination methods that are used in literature. From a quick read of the book, it doesn't look like this content is explicitly covered. My feeling is that it would fit into the theoretical background section right after signal decay.

See this repo for the content: https://github.com/jsheunis/multi-echo-combination.

Thoughts?

[tsalo]

I definitely think it's worth it to go into more detail about optimal combination than we currently do, and that seems like a great place to put it!

[jsheunis]

Great, will start working on a PR. My content is mostly just theory and equations, so I'll ask reviewers to also contribute some of their own tips/experiences wrt specific combination methods, to provide some practical context too.