Improvements to merging duplicates

[bakhtos]

Fixes #8 Fixes #23 Following changes are implemented:

• Both services and external components now store a field name, instead of services having servicename, which simplifies parsing of the output/allows to reuse code for service on external components and vice versa
• Merging functions now accept one collection at a time and are called several times in perform_analysis() to reduce code duplication (e.g. merge_service was called with both services and external components at the same time, but the processing code for each was almost identical and could be reused)
• Issue #8: the reason why external_component was duplicated was because this block was concatenating the type property; now it skips it https://github.com/tuhh-softsec/code2DFD/blob/f0959c61a649cc3b2de0b6614b0cf8ff88bdeb13/core/dfd_extraction.py#L474-L478
• Merging functions are simplified, using itertools to create all possible pairs for comparison
• Issue #23 : name, sender and receiver fields now have str.casefold() applied to them in the merging functions, which removes duplicate services as described in the issue
• clean_database_connections() is now applied after merging, since it compares receiver to name
• As issue #24 shows, there is a chance that a port is something like 3306/tcp, this is taken into account when merging annotations

[SimonAdrian]

Changes look good, the simplifications are good and the fixed issues valuable. Thanks!

[SimonAdrian]

I had issues with the VSCode plugin and the result was another PR that duplicated yours. Your changes are merged now.