Added DefaultContactMat in order to have a standard interface model to create internally for the contact implementation to use.
Added method to compute dNdxi on an edge, so far only implemented for FEI2dQuadLin.
Added a default contact material with a penalty stiffness in the normal direction only. Mainly intended for internal use in the contact implementation.
Major updates in the contact implementation. Each contact element will have a ContactPair object which keeps track of all the info about slave nodes, master edges and so on.
It is mainly a helper object andshould be responsible for computing closest point projections, evaluating base vectors etc.
Added first version of Node2Edge implementation which seems to work ok.
Modifications to contactPair and Node2Edge in order to generalize things for future elemnts in order to minimize code duplication.