Deformation picard

Picard method is now implemented for small deformation processes, restricted to linear elasticity. The 'assemble' method is basically the same as the 'assembleWithJacobian', only one or two lines are different. However, I decided to keep the two methods appart rather then combing them, since it is more convenient and easier to read this way.

In order to assure that inelastic or non-linear elastic models are used, I came up with a flag in SmallDeformationProcessData which is set to true for linear elastic materials in the process creation routine. This is not the best way (maybe its the worst) to check for the constitutive relation. I think, this flag (or some kind of a type-specifier) is needed in the material object itself, since it belongs to the material and not to the process (BTW: the same counts imo for the solid density). But, this would require much more changes; tell me what you think about it..