Merged patch 1354 from trunk:

Make data accmulation work again.
Before all indices with the same attribute attached on both sides were
skipped. (Bug was introduced during the periodic boundary refactoring.)

[[Imported from SVN: r1355]]

.gitignore

+Makefile
+Makefile.in
+config.*
+configure
+dependencies.m4
+aclocal.m4
+autom4te.cache
+depcomp
+install-sh
+missing
+mkinstalldirs
+libtool
+dune-istl.pc
+semantic.cache
+configure.lineno
+stamp-h1
+dune-istl-*.tar.gz
+dune-istl-?.?
+ltmain.sh
+am
+.libs