Add isCircularReference to properties

[eriktim]

As a preparation to a major refactoring of the Elm generator I would like to add the ability of detecting circular references. Support for self referring types was already in place, but this PR also detects types that indirectly refer back to themselves. Every property that indirectly refers back to itself gets marked with isCircularReference.

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cc @wing328@jimschubert@cbornet@ackintosh@jmini@etherealjoy

[wing328]

Can we add this to the default codegen instead as users would expect isCircularReference with a valid value (true, false) for all generators?

[eriktim]

@wing328 that was my intention, that's why I added it to updateAllModels in DefaultCodegen. Somehow this did got not triggered in Elm, so I thought I'd probably overridden it. Is updateAllModels the correct place to put this?

[wing328]

Can you please move it to updateAllModels and I'll run some tests later to troubleshoot?

[eriktim]

@wing328 I already put it in updateAllModels. Do I need to remove it from the Elm codegen?