Abstract stress calculation from integration point

[ahojukka5]

In the integration point, we need to have a place for a material model and we need to define a way to ask a stress state and jacobian from there given some strain or displacement. It should be abstract presentation enough so that we can have very different approaches for material modeling, e.g. microscale models added to integration point. In the same spirit as in #43, we could then have a possibility to override or extend a stress calculation for some particular integration points.