* unary intersection integrands are applied to one test basis on one side
  of an intersection, meant for functionals
* binary intersection integrands are applied to one test and one ansatz
  basis on only one side of the intersection, meant for bilinear forms
* integrands can derive from multiple interfaces, we need to provide an
  additional combined interface for each combination for operator +
* one can now use with_ansatz(u) to obtain integrands depending only on
  the test basis