Different number of coefficients for each integral

Different number of coefficients for each integral, to avoid overhead in tabulate_dof_map and filling of w[][] when there are many separate coefficients. Can quickly grow complicated though.

Alternative:
     class *_integral
     {
       virtual bool needs_coefficient(uint i) const = 0;
     }
 This can return true for all coefficients it uses, such that when assembling a set of integrals the coefficients neither of them needs can be skipped.