Fix grayscale image conversion and watershed mask error in segmentation analysis notebook.

[haesleinhuepf]

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The updates made to address issue #33 involved enhancing the segmentation workflow in the Jupyter notebook by adding preprocessing steps to convert images to grayscale for consistent handling. To resolve errors occurring in the segmentation.watershed function due to shape mismatches between the markers and the mask, a new boolean mask masked_image is created from the grayscale image, ensuring proper execution without shape errors. These modifications improve the robustness and accuracy of the cell segmentation process.

During solving this task, the following errors occurred:

• Error during {'action': 'modify', 'filename': 'notebooks/segmentation_analysis.ipynb'}: 409 {"message": "notebooks/segmentation_analysis.ipynb does not match e8214a957991fc35336ca6918d76ba221398fb2f", "documentation_url": "https://docs.github.com/rest/repos/contents#create-or-update-file-contents", "status": "409"}

      Traceback (most recent call last):
    File "C:\structure\code\git-bob\src\git_bob\_ai_github_utilities.py", line 395, in solve_github_issue
      message = filename + ":" + create_or_modify_file(repository, issue, filename, branch_name, discussion,
                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "C:\structure\code\git-bob\src\git_bob\_ai_github_utilities.py", line 301, in create_or_modify_file
      execute_notebook_in_repository(repository, branch_name, filename)
    File "C:\structure\code\git-bob\src\git_bob\_github_utilities.py", line 758, in execute_notebook_in_repository
      repo.update_file(file.path, commit_message, new_notebook_content, file.sha, branch=branch_name)
    File "C:\Users\rober\miniforge3\envs\devbio-napari-env\Lib\site-packages\github\Repository.py", line 2541, in update_file
      headers, data = self._requester.requestJsonAndCheck(
                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "C:\Users\rober\miniforge3\envs\devbio-napari-env\Lib\site-packages\github\Requester.py", line 550, in requestJsonAndCheck
      return self.__check(*self.requestJson(verb, url, parameters, headers, input, self.__customConnection(url)))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    File "C:\Users\rober\miniforge3\envs\devbio-napari-env\Lib\site-packages\github\Requester.py", line 611, in __check
      raise self.createException(status, responseHeaders, data)
  github.GithubException.GithubException: 409 {"message": "notebooks/segmentation_analysis.ipynb does not match e8214a957991fc35336ca6918d76ba221398fb2f", "documentation_url": "https://docs.github.com/rest/repos/contents#create-or-update-file-contents", "status": "409"}
  

closes #33