add to linter rules DFD inconsistencies

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Examples:

• No trust boundaries present (there should be at least one trust boundary)
• Process element lacks entry or exit dataflow (there should be an entry and an exit)

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I was looking around for "DFD grammar" to see if there was a more formal set of DFD rules such that one could write a parser in order to flag issues like the above, and came across this: https://arxiv.org/pdf/1011.0278.pdf

See for example, definition 9 from that paper:

Definition 9: Syntax rules of data flow diagram: • At least one input data flow for a process • At least one output data flow for a process • Process from external entity cannot move directly to another external entity • At least one input data flow for a data store • At least one output data flow for a data store • Data from one data store cannot move directly to another data store