Fix failed Stream rotation in-place modification

[gbbofh]

What does this PR do?

This PR fixes erroneous behaviour in Stream.rotate -- namely that when a rotation fails on a Stream object, the relevant Trace objects are removed from the stream. This fix alters this behaviour such that if Stream.rotate fails in the process of rotation, the relevant Trace objects will not be removed. In addition, it modifies the test_rotate method of the Stream testing suite to ensure that erroneous rotations keep the same size when rotation fails.

Why was it initiated? Any relevant Issues?

This PR was initated in response to Issue 2692

PR Checklist

• Correct base branch selected? master for new features, maintenance_... for bug fixes
• This PR is not directly related to an existing issue (which has no PR yet).
• If the PR is making changes to documentation, docs pages can be built automatically.
  Just remove the space in the following string after the + sign: "+ DOCS"
• If any network modules should be tested for the PR, add them as a comma separated list
  (e.g. clients.fdsn,clients.arclink) after the colon in the following magic string: "+TESTS:"
  (you can also add "ALL" to just simply run all tests across all modules)
• All tests still pass.
• Any new features or fixed regressions are be covered via new tests.
• Any new or changed features have are fully documented.
• Significant changes have been added to CHANGELOG.txt .
• First time contributors have added your name to CONTRIBUTORS.txt .

[trichter]

I think you based your modifications on the maintenance branch. If you want to merge into master your commits need to be rebased on master.

[gbbofh]

@trichter Thanks for pointing that out -- I intended to merge into maintenance but it appears that I forgot to select the correct branch.