fix memory leak

[psychocoderHPC]

With #44 we tried to fix the memory leak which can happen if more than one thread is allocating a new chunk. The PR #44 was not fixing the issue and introduced a double free issue.

• [ ] PR must be tested
  • [x] examples work on @psychocoderHPC laptop
• [x] this PR contains #47 else testing is not possible

@michaelsippel I have not tested the PR and wrote it blindly, we need to perform a runtime test before merging.

[psychocoderHPC]

@michaelsippel I applied your review comments and rebased against #47

[psychocoderHPC]

generally there is a confusion between 'chunks' & 'items' in the variable naming & comments. Otherwise I have a commit that refactors the Chunk such that it stores direct pointers instead of indices and we should build on that to implement the fix

I am fine if we refactor these parts in a follow up but as long as there is no PR fix that instantly fixes the current issue of memory leaks I would like to bring it in to have a version that is working correctly.

I am also fine if this PR is not going in, both ways are possible as long as the dev branch becomes usable again.

[michaelsippel]

For testing we should use small chunk sizes with many tasks since otherwise when everything fits into one chunk, the implementation will not be stressed

[psychocoderHPC]

Unfortunately, this PR creates a deadlock :-( during testing

[michaelsippel]

now already included in #48