Simplified interface for stress/tangent updates

Provide a simple, robust and extensible interface for constitutive updates with internal variables. Tasks include:

- Getting quadrature data from dolfinFunctions

- Getting/setting quadrature data (use STL objects, ie. std::vector (including vector of vectors))

- Use named variables, e.g. double = data["equivalent plastic strain"]. Permit users to add named data to maps.