[16.0][IMP] mis_builder: allow to export multiple report in one xls

[AnizR]

Description

Allow to export multiple reports at once.
In one '.xls' where every report is a sheet.

To do so, I added one action 'Export XLS' visible on tree view of mis_report_instance.

[OCA-git-bot]

Hi @sbidoul,
some modules you are maintaining are being modified, check this out!

[sbidoul]

This looks good at first sight.

If we don't have test coverage for this, it might be a good occasion to add some.

[AnizR]

If we don't have test coverage for this, it might be a good occasion to add some.

There's already a test that checks that it is possible to generate a xlsx report. I've added a small one for generation of a report over 2 instances.
Note that it doesn't check the content but, there are tests on the computation of the report and its values.

[github-actions]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days.
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