Added support for writing XLSX

[henka-rl]

No description provided.

[jam01]

Thanks @henka-rl I'll take a closer look, but this is a great addition.

Would it be possible to support reading as well?

Only thing so far is that we support input streams for reading, and output streams for writing, so we'll need to address that.

[henka-rl]

The library I found for XLSX only supports writing and in my current project we don't have a requirement for reading XLSX so at the moment I don't have any time for implementing reading unfortunately.

Regarding OutputStream for writing in the other FormatPlugins. The OutputStream is created inside the write method and at least in Camel (where I use Datasonnet) this OutputStream can't be handled. But in DefaultXLSXFormatPlugin I return an InputStream containing what has been written to the OutputStream and this works well in Camel.

[jam01]

Camel will take care of converting from one type of stream to another. In general we'll need to change this to OutputStream, which is not a big change. However, right now I'm a little more interested in the implications of using this sett4 library. It's not hosted in maven central and doesn't seem very widely used. Perhaps we can use Apache POI directly, or bring in some of this code into datasonnet-mapper since it's also Aapahce 2.

[henka-rl]

I've tried using the OutputStream from Datasonnet in Camel and it fails. I don't see how you ever could read from an Outputstream.
The sett4 library was the only Jackson implementation I found for XLSX and as it builds on CsvMapper it was easy to implement it in Datasonnet as I just (more or less) needed to copy DefaultCSVFormatPlugin. This library isn't perfect. It lacks support for reading and you can't set the name of the sheet. But it solved the requirement I had...