ReadSDMX Roxygen and Eurostat Example

Added an example of Eurostat data to the inst/data. As Eurostat provides SDMX2.0 only via SOAP I added the file locally. REST is only available for SDMX 2.1. The file is used from the readSDMX examples

In addition I noted that moving the documentation to Roxygen is on the issues list, so I did this change for readSDMX.R