Restructured ISTL documentation.

istl/basearray.hh

istl/bcrsmatrix.hh

istl/bvector.hh

istl/io.hh

istl/istl.hh

istl/matrix.hh

istl/operators.hh

istl/owneroverlapcopy.hh

istl/paamg/aggregates.hh

istl/paamg/amg.hh

istl/paamg/construction.hh

istl/paamg/galerkin.hh

istl/paamg/graph.hh

istl/paamg/hierarchy.hh

istl/paamg/indicescoarsener.hh

istl/paamg/transfer.hh

istl/preconditioners.hh

istl/remoteindices.hh

istl/scalarproducts.hh

istl/schwarz.hh