Introduce a first creational test for edges in Netwon Raphson context:
this induce a refactoring to the code, especially for builder code,
since we move the Switch on/off state into the edge instead of belonging
to builder.
Add a new test to check the creation of a switched off edge.
Refactor the edge class to delegate the computation for covariant and
controvariant little Ks to the switch state object.
Introduce a test class for the computation of little Ks.