Make the local_to_global functions in the ConstraintMatrix even more efficient by...

Make the local_to_global functions in the ConstraintMatrix even more efficient by pre-sorting all the rows and columns that are going to be written into the sparsity pattern / sparse matrix. This makes the values added for each element really unique, which is faster. Moreover, the matrix and sparsity pattern classes can now take an additional argument sorted when adding many elements, and some of them already make use of the sorted-ness. For block matrices, the sortedness is used in an even more clever way by writing directly into the blocks (even though I had to introduce a lot of duplicate code to be in line with the templates).

git-svn-id: https://svn.dealii.org/trunk@18512 0785d39b-7218-0410-832d-ea1e28bc413d