Extend FeatureExportExtension interface

[yaozhihang]

It would be helpful to add two methods somehow like exportStart() and exportComplete() into the interface FeatureExportExtension. For example, if we write a plugin to do post-process and log the processed object into a separate CSV file, we could create a file writer within the method exportStart() and close the writer in exportComplete().

https://github.com/3dcitydb/importer-exporter/blob/master/impexp-core/src/main/java/org/citydb/core/plugin/extension/exporter/FeatureExportExtension.java#L35

[clausnagel]

Yes, sounds like a good idea. I guess the interface would also need to handle cases where the export is aborted by the user or even terminates abnormally. Because if you open resources when the export starts, you should close them even if the export process fails.

[yaozhihang]

True, nice remark.

[clausnagel]

fixed with #200