[fluidmatrixinteraction] Temporary fluidmatrix law objects always get copied

When using

    template<class ElementSolution>
    auto fluidMatrixInteraction(const Element& element,
                                const SubControlVolume& scv,
                                const ElementSolution& elemSol) const
    {
        const auto params = ....; 
        return makeFluidMatrixInteraction(MyTemporaryPcSwCurve(params));
    }

the temporary object gets always copied in the PcKrSw constructor in fluidmatrixinteraction.hh

This also prevents using unique_ptrs in MyTemporaryPcSwCurve.

I suggest to change PcKrSw's ctor such that:

    template<class CtorType>
    PcKrSw(CtorType&& impl) : impl_(std::forward<CtorType>(impl))
    {
        static_assert(std::is_same_v<std::decay_t<T>, std::decay_t<CtorType>>);
    }

This means that CtorType (please feel free to suggest a better name) can be different from T.