Fix #605: make background_array optional when skip_classification=True

[KpradeepKumar25]

Background_array was always required even when skip_classification=True. This fix makes it default to None and adds
ValueError with a clear message when classification is attempted without a background array.

Fixes #605

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Description

What is this PR

• Bug fix
• Addition of a new feature
• Other

Why is this PR needed?

background_array was always a required argument even when skip_classification=True. This blocked researchers who only have
a single channel (signal only) from using cellfinder for detection without classification.

What does this PR do?

• Makes background_array default to None instead of being required
• Adds a clear ValueError message if someone tries to run
  classification without providing background_array
• Updates the docstring to reflect that background_array is optional

References

Fixes #605
Related to #352

How has this PR been tested?

Manually verified that calling main() with skip_classification=True and no background_array no longer raises a TypeError.
Also verified that calling main() with skip_classification=False and no background_array now raises a clear ValueError.

Is this a breaking change?

No. Users who already pass background_array will see no change. Users who pass skip_classification=True without background_array will now get the expected behaviour instead of a TypeError.

Does this PR require an update to the documentation?

The docstring for background_array in main() has been updated to reflect it is now optional.

Checklist:

• The code has been tested locally
• Tests have been added to cover all new functionality (unit & integration)
• The documentation has been updated to reflect any changes
• The code has been formatted with pre-commit