Updated Excel Transform to allow streaming where a template is used

[dameikle]

When using the Excel Transform in streaming mode (i.e. XLSX and using SXSSF in POI) to handling the generation of large Excel documents, where we used a template the transformation would always use XSSF and fail.

This update allows streaming to happen where a template is used though has all the limitations of POI in this mode so is ideal for adding new content to a template, and not for amending existing content.

Given this is a change in behaviour from before which silently fellback to XSSF mode, was debating logging an info message on this but didn't as the transform would fail.

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[hansva]

Thanks for the contribution @dameikle , sorry about the delay for the merge!