Support set-valued functions

*Originally reported by* **Pieter Collins (Bitbucket: [pietercollins](https://bitbucket.org/pietercollins), GitHub: [pietercollins](https://github.com/pietercollins))**

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Add full support for set-valued functions R^m->P(R^n).

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- Bitbucket: https://bitbucket.org/ariadne-cps/development/issue/34
