colab: make setup cell more robust

[Masood-Akram]

Improved Colab detection and made the clone step idempotent (won’t reclone on rerun).
Prevents false negatives and improves notebook re-runnability in Colab.

[Masood-Akram]

This PR makes the Colab setup detection more robust and avoids re-cloning on reruns. Happy to adjust to preferred style.

[zivy]

Hello @Masood-Akram,

Thank you for your contribution. This improves the original code:

1. More robust checking if on colab via importing google.colab instead of checking for environment variables.
2. Shallow clone vs cloning the whole history.

What is the motivation for the dedicated _in_colab function vs. a more direct approach shown below?

try:
  import google.colab
  import pathlib
  # If the directory does not exist, perform a shallow clone (latest commit no history).
  if not pathlib.Path("SimpleITK-Notebooks").exists():
    !git clone --depth 1 https://github.com/InsightSoftwareConsortium/SimpleITK-Notebooks
  %cd SimpleITK-Notebooks/Python
  !pip install -q -r requirements.txt
except Exception:
  pass # not on colab

If the above code is acceptable to you over the usage of _in_colab, please modify your PR accordingly, otherwise would appreciate it if you could motivate the use of _in_colab.

[Masood-Akram]

Thanks for the review. I added _in_colab() mainly to keep the Colab detection logic separate from the setup steps (readability and easier reuse across notebooks). That said, I agree the inline try/except approach you suggested is simpler and consistent with the style in the repo. I can update the PR to use your direct approach.

[zivy]

@Masood-Akram Thank you for the contribution. Merging and will propagate to the other notebooks that support colab.

[Masood-Akram]

@zivy Happy to contribute to this repository.

[zivy]

@Masood-Akram, just an FYI, I propagated this change to all the notebooks that use colab and listed you as co-author (PR #499).

[Masood-Akram]

@zivy Thank you very much.