Implement a parsable functor-expression printer

implements #8 (closed)

I currently insert braces everywhere, meaning that printParseableFunctor . parseFunctor = id should hold, parseFunctor . printParseableFunctor = id most certainly does not.

Example output:

{n, f}xPX -> {n, f}x(PX)

D({f,n}xPX) -> D({f, n}x(PX))

D({f,n}xP(DX)^{a, b, c}+({f, n}xX + (Z, min)^({f ,n}))) 
-> D({f, n}x(P(DX))^{a, b, c} + ({f, n}xX + ((Z, min)^({f, n}))))