New evaluate_integrand function

I propose adding a new function called evalute_integrand to each of the integral classes. This gives users flexibility to build applications where integration must happen on a non-standard domain (like a part of a triangle).

The signature of the evalute_integrand function would be (in the case of the cell_integral class):

   /// Evaluate integrand at given point
   virtual void evaluate_integrand(double* A,
                                                    const double * const * w,
                                                    const cell& c,
                                                    const double* coordinates) const = 0;

Alternatively (or in addition) we might want a function that does the quadrature for a given set of points and weights:

  virtual void tabulate_tensor(double* A,
                                              const double * const * w,
                                              const cell& c,
                                              uint num_quad_points,
                                              const double * const * coordinates,
                                              const double * weights) const = 0;

This would be good to add in UFC 2.0 (together with other changes listed as blueprints).