Section 3 revision

I have 'gone through' section 3 reorganizing it and adjusting quite a lot of the text. 
I have added a preliminary version of architectural implications. There is likely to be further 'stuff' in this section.

Occasionally, I felt that a slightly radical edit was required. In that case I kept the original text but with strike-through.

Some diagrams will need to be updated. I have left comments where I could to signal the required change.

Frank