Adding point integral class

MER: The suggested interface looks good -- I've reimplemented this (easier b/c of flurry of ufc changes since original branch). In addition to the generic point, I suggest we add a function for local vertices as below. We can generate significantly more efficient code for that, and it is a common use case.

    /// Tabulate the tensor for the contribution from a local vertex
    virtual void point_integral:: tabulate_tensor(double* A, const double * const * w,
                                                                                    const cell& c,
                                                                                    const std::size_t vertex) const = 0;

MER: I've added the above, with tabulate_tensor both for a set of arbitrary points (relative to a cell) and for a local vertex index now.