Comments and edits for chapter 6.1

[clausnagel]

As discussed in our last subgroup meeting, here is a PDF with some comments and edits for chapter 6.1.
21-006.pdf

[TatjanaKutzner]

I did some of the edits directly in the asciidoc files, but for other things I only added answers to your comments. A short discussion of these things would be helpful.
21-006_TK.pdf

[PeterParslow]

In the meeting just now, you showed a 'global requirement' which we then discussed. I can't find this in either 21-006.pdf, 21-006_TK.pdf or 21-006.adoc, so I'm resorting to an issue comment rather than a pull request.

First of all, why GML 3.2.1 rather than 3.2.2? I don't think that should make any difference.

Should we have an identified 'global requirement' like

"XML instance documents claiming conformance to this specification shall validate with respect to the schema files."

Then "Note: this means that they have to be valid GML 3.2.1 or GML 3.2.2 documents. They may use certain aspects of CityGML 3.3; see Annex X for further information."

As I've noted on the CodeList issue, we can't state that CityGML's GML schemas conform to CityGML 3.3 because we use the codeType which has been deprecated in CityGML 3.3 for referencing code list entries.

[PeterParslow]

I have moved this comment into the sub-group working file. Thanks Tatjana for pointing me to that.