Add statement about LCG version to manual

The build system of the framework provides a set of automated tests which are executed by the CI to ensure proper functioning

of the framework and its modules.

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Since different Geant4 versions may result in different events, a specific Geant4 version needs to be chosen for the

test suite. All tests in the repository are written for Geant4 11.0 / LCG102.

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The tests can also be manually invoked from the build directory of Allpix Squared with:

```shell