#1418 Modified the CMakelists used for the test so that the several libraries builds have the expected behaviour regarding the abstract factory idiom.

#1418 CI test for the microsphere operator, in the same spirit as the ones used for the ModelInstances.

#1418 Clean up the test to use the bare minimum of operators to reproduce the Matlab reference case.

#1418 Fix conversion warnings and add a model used for the validation of the operator related to the microsphere.

#1418 Aggregate several commits together... to get one that actually compile! There was an issue of includes not put properly at the right time in the repository, and also the fact I moved from Seldon to Xtensor but Jerome worked (rightfully) on top of previous master. Most of the content of the resulting commit is Jerome's, except the ppart related to Xtensor.

Below I put the text of the original commits:

1 - PartialSpecialization for the new Microsphere model used for the InternalVariablePolicy.
2 - Moved MicrospherePolicy dependencies into the CoreLibrary.
3 - Parameter description updated.
4 - Update XCode project: during rebasing with main branch, there were conflicts that were solved by keeping this main branch project; new files have been properly added in this commit.

#20 Extend the CheckInvertedElement to properly handle the case it was not defined in the InputData (the singleton is created but any attempt to use it will result in an exception).

#1496 Deactivate Verrou: not yet ready (I have to figure out how to compile properly Valgrind to work in Docker images no matter what is the underlying architecture of the VM).

#1490 CI Sonarqube: Fix an issue when running it on the main project (using CI_PROJECT_NAMESPACE was not accepted by SonarQube...) and also updating path to includes for third-party libraries.

#1498 CI Yaml: create new Yaml files for build and tests (and comment them as I want to debug Verrou issues).

#859 Increase massively low timeouts in tests: by experience many may fail if for some reason the system is busy when the job is attempted.