Fix external distance function

[arrjon]

This pull request improves the robustness of the distance method in pyabc/external/r/r_rpy2.py by adding stricter validation of the output from the R distance function. Now, the code checks that the result is a single numeric value and raises an informative error if it is not.

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Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.62%. Comparing base (fafff1d) to head (00b1c7b).

Files with missing lines	Patch %	Lines
pyabc/external/r/r_rpy2.py	0.00%	4 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #654      +/-   ##
==========================================
- Coverage   80.64%   80.62%   -0.02%     
==========================================
  Files         141      141              
  Lines        8530     8533       +3     
==========================================
+ Hits         6879     6880       +1     
- Misses       1651     1653       +2     

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Pull request overview

This pull request enhances the robustness of the R distance function wrapper by adding validation to ensure the R function returns a single numeric value. The validation checks the shape and size of the returned array and raises an informative error message if it doesn't meet the requirements.

Key Changes

• Added validation logic to check that R distance functions return a single numeric value
• Replaced direct float conversion with array validation using np.asarray().squeeze() and size checking
• Improved error messaging to include the function name, expected return type, and actual shape/size received

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