Reading CityGML fails if file has a <cityObjectMember> only containing ignored elements

[mlavik1]

When parsing a node we always create a new CityObjectElementParser.

This causes an exception to be thrown in CityGMLElementParser::endElement (see citygmlelementparser.cpp) if the only contains children that are ignored (because they haven't been implemented, etc.).

I've created a test file (modified version FZK-Haus) that shows the problem:
ignore-test.zip

This is the model where I first encountered this problem (because it uses "grp:CityObjectGroup", which is not implemented):
LoD3_Railway.city.zip
I've encountered the same issue on other models too.

I suggest replacing that error with a warning, and instead handling this case. I can create a PR for that. Though, please correct me if you think that would be a bad idea 😅

In the case of a container element containing a child element with an xlink, it should also be safe to continue parsing - but it's fair to print a warning, since that element won't be parsed. I noticed once case where that happens (tran:intersection with an xlink - which isn't handled at the moment). I might make a bug/PR for that too.

And by the way, thanks a lot for making this amazing library!