sysl does not generate the right relationship with clashing fields

[nofun97]

The relationship data structure in diagram generation does not allow multiple relationship types with the same diagram.

For example:

App:
    !type VeryComplex:
        Complex <: sequence of complex
        Simpler <: complex

    !type complex:
        simple <: string

would generate the following diagram:

Expected behavior:

[nofun97]

Changing the relationship data structure will most likely break compatibility with sysl-catalog