Feature/operators in dofmatrix
This branch moves the assembling to DOFVector and DOFMatrix. Also, the operators are associated to the vector and matrix directly. This allows to have a more precise initialization of the nz structure. Also the assembling of vector and matrix is separated. Additionally, a matrix operator can be assembled directly into a matrix.
With this change also the assembling time could be reduced and the procedures for assembling are cleaned up, i.e. only the already bound localView is passed to the assemblers.
TreeData no stores a pointer to the basis instead of a pointer to the tree. This is more usable, since the tree can be accessed only via the localView that is not stored by reference but by value. This the pointer is invalidated after construction and needs a additional storage of the localView object, just for the access of the tree.
Todo: also the dirichlet bc should be moved to matrix and vectors side. But since it depends on both, matrix and vector, this interaction must be designed carefully. This will be done in a separate MR.