merge graph edges

[nineinchnick]

Allow to collapse a request and response dataflow into a single double-arrow edge. This greatly simplifies bigger DFDs and makes them more (actually) readable.

Added 3 new Dataflow attributes:

• isResponse - bool, when set and there's exactly one other dataflow for the same source and sink in reversed direction treat it as the request
• responseTo - point to another dataflow that this one is a response to
• response - point to another dataflow that's a response to this one

If only one of those attributes is set, others are filled in automatically, when possible.

When TM.mergeResponses is set, the resulting DFD would look like:

Possibly closes #53

[ghost]

DeepCode's analysis on #b9aa55 found:

• :x: 0 critical issues. :warning: 0 warnings and 1 minor issue. :heavy_check_mark: 0 issues were fixed.

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[izar]

Wow, I don't know. To me it makes it harder to read. Let me ask the rest of the team what they think.

[nineinchnick]

The example might be too simple. It works better for large models, especially when there are multiple data flows between one pair of assets. Also note this is optional and not enabled by default.