PivotTable exports: sheet needs title

[nursix]

When exporting a pivot table report as XLSX, the work sheet lacks a meaningful title (it's simply called "Sheet" instead):

The sheet title should reflect the chosen (aggregated) fact, e.g. "Number of Cases", or it could be the report title if configured as a named report. As a fallback, the sheet title could be simply "Report".

[NA-V10]

I'd like to work on this issue.

My plan is to use the report title if available, otherwise fall back to the selected fact label (e.g. "Number of Cases"), and finally use "Report" as a generic fallback. I'll also make sure the worksheet title is sanitized for Excel sheet name restrictions.

[nursix]

Fixed by #84