[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.
If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.

[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.
If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.

[AnizR]

@sbidoul, I'll soon port this PR in 19.

Do you prefer to try have some review here to have it merged then, migrate to 19 or close this PR and open a new one in 19?