Improve Issue Details

[gmuecke]

Current issues contain a default description from the MutationOperator definition which is the same for all issues created for the same MutationOperator rule.

Pitest however provides more details on each individual mutation providing a clearer description of what has actually been mutated. This information is available in the mutation.xml in the element.

This element has to be processed and used as violation description. If no such description is present, the default description from the mutation operator definition should be used as fallback.