[DOC]: Merge and concatenate chapter for user guide

This adds a chapter on merge and concatenate to the Iris user guide. The chapter describes what merge and concatenate do, the differences between the two and hence the situation in which each should be used. The chapter also contains a section on issues commonly encountered with merge and concatenate and, where possible, provides the solution to avoid the problem occurring.
The primary purpose of this document is to be descriptive rather than friendly. Part of making it descriptive is repetition - the section on concatenate is quite deliberately a duplicate of the section on merge. 

Also, this userguide is designed to be dipped in and out of - I would fully expect people to just read the section on concat and completely ignore the section on merge, because they only need to learn something about concatenate. At this point connecting this section to a section they've neither read nor care about becomes a distraction...