Serialisation of nested types

[jordangray]

Allow nested types to be serialised in one of two ways:

• flat—add extra columns with headings on the same line as properties of the containing type; and
• subheadings—nested types are serialised such that their headings appear on a separate line below the containing type's headings. The number of heading lines should be unlimited, cells above should be merged and given the heading name corresponding to the property on the containing object. FreezeHeaders will freeze all heading lines.