Support for hierarchical Excel data sources

[nils-christian]

Currently the ExcelDataSource annotation supports only flat data structures. However, in some cases you want to build a larger and more complex (hierarchical) data structure which has still to be mapped from an Excel row. At the moment one has to implement this manually. It would be possible to generate this as well though. This could be achieved in two ways:

1. The generated data source maps the fields of the sub-datatypes as well.
2. The generated data source delegates to the data sources from the sub-datatypes.

Both ways would require that one can inform the generator that the field has to be handled in a specific way instead of just delegating to the conversion service. A possible candiate would be the ExcelColumn annotation which is usually used to specify a header name. We could add another field to the annotation as a marker for the generator.