Release/xml import

[eyck]

This PR adds 2 extensions thus addressing https://github.com/fvutils/pyucis/issues/16 :

• import an UCIS XML including the per_instance and merge_instances property
• adds update_coverage() method to the CoverageReportBuilder and modif report accordingly. This method creates the union-merged coverage for instances having the merge_instances property set to true. The reporting uses the merge_instances and per_instance properties to calculate the coverage for reporting.

The implementation shall be seen as proof-of-concept. Naming can be improved and maybe it is not the right location. Maybe it is more a database functtionality rather than a report builder function.

[eyck]

If this is acceptable I can work on fixing and extending the CI tests