Make sure JOMDoc respects the OMDoc document model

[clange]

I rediscovered chapter 22.2 of the OMDoc spec, which specifies an "OMDoc document model", i.e. something like DOM for generic XML, plus a few OMDoc-specific things, e.g. that for some elements the order of their children does not matter. JOMDoc should respect this model. I.e. it should be checked whether the current implementation does.

[kohlhase]

I consider chapter 22.2 of the OMDoc spec outdated by new technology, in particular Normen's and Serge's equality stuff in general and S"onke's work on an OMDoc document model in particular. So I do not expect the document model material to survive into OMDoc 1.6. I am not sure what to do with that in OMDoc1.3, maybe we can re-say the material in a way that is more compatible with the new technology.

After that caveat about the future, you are right, we should make sure that the published document model is followed. But that should not be a big problem, since it is so very weak.