histogram: check for duplicates when merging

Improves performance because it's not necessary to caculate the length
of the histogram on each insert.

git-svn-id: https://scalaris.googlecode.com/svn/trunk@6570 71766da9-9752-0410-ab17-bfafa751c56a
histogram: remove find_largest_window

The function is not needed anymore and was primarily built for
educational purposes.

git-svn-id: https://scalaris.googlecode.com/svn/trunk@6571 71766da9-9752-0410-ab17-bfafa751c56a
histogram: correctly merge duplicate values

- return the position in find_smallest_interval instead of the value
  which may not be unique

- merge the smallest interval using a position

git-svn-id: https://scalaris.googlecode.com/svn/trunk@6572 71766da9-9752-0410-ab17-bfafa751c56a