Split files into smaller "packages"

[bensteUEM]

at present multiple files are >1000 lines. ideally sorting through these files should result to a better readability

• [x] SngFile.py could use inheritance
• [x] test_sng.py could be split by function similar to SngFile
• [x] main.py could be split into multiple files covering different use cases
• [x] test_main.py could follow main.py

[bensteUEM]

ignoring main.py and respective test because it should be converted to #22