Add functionality to merge relative opossum files

[meretp]

The CLI tool should be able to generate one opossum file including multiple opossum files and reflect their relative position in the file tree. E.g. if I run the command merge-relative ../OpossumUI/example-files/opossum_file1 /test/opossum_file2 the file tree should like like the one in the picture below

The --merge-relative command in the haskell tooling serves as an example for this functionality.