Export the spmatrix as scipy sparse matrix for scheme.assemble

Make it possible to extract dof vectors and assembled matrices for different backends (fem,eigen,istl,petsc)

• added a method as_* to the discrete function (as_numpyfor eigen/fem,as_istlfor istl andas_petscforpetsc`
• scheme.assemble returns a scipy crs matrix for eigen/fem, an istl bcrs or a petsc matrix depending on backend used

To do: at the moment there is a method space.numpyFunction to generate a discrete function from a given numpy array. We need something similar for BVectors and Petsc::Vec storages. Possibly:

space.discreteFunction(vector,name)

and the correct version is used depending on the type of vector. Alternative, the vector has to match the storage provided by the space.