SampleList: fix IO bug
Previously, if a SampleList that contains n samples has already been saved and afterwards a SampleList with n-m (m>0, n-m>0) is saved and then loaded again, it will consider the samples between n-m and n as part of the new, updated SampleList.
This patch adds a fix and tests that make sure that this faulty behaviour does not happen again in the future.
@mtr @veberle a review (and eventual merge) is highly appreciated :)