Make intermediate file an XLSX instead of CSV

[ursjoss]

This will resolve a few open issues:

Pros:

• current problem that date is expected to be dd.mm.yyyy. dd.mm.yy has to be manually corrected before continuing the process
• diffing on file level is possible, but once saved as CSV from Excel, next to the data format, also the number format is contorted, e.g. 0.0 is saved as 0, depending on the number format, the digits after the decimal point may even be rounded, what not only obfuscates the diff but may even change the content.

Cons:

• diffing on file level will not be possible

Steps:

• have the prepare task create an XLSX instead of CSV
• have the export task expect an XSLX istead of CSV

If it is cheap to support both CSV and XLSX (with XLSX as default), prefer that.