Fix median sampling error if only 1 distinct value

[michael-nml]

This commit addresses an exception that occurs during calculation of the median sampling error if the data containts only 1 distinct value.

Error being addressed:

The data appears to lie in a lower-dimensional subspace of the space in which it is expressed. This has resulted in a singular data covariance matrix, which cannot be treated using the algorithms implemented in gaussian_kde. Consider performing principle component analysis / dimensionality reduction and using gaussian_kde with the transformed data.

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 78.41%. Comparing base (7da83e7) to head (4816d95).

Files	Patch %	Lines
nannyml/sampling_error/summary_stats.py	50.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #377      +/-   ##
==========================================
- Coverage   78.44%   78.41%   -0.04%     
==========================================
  Files         110      110              
  Lines        8514     8520       +6     
  Branches     1514     1514              
==========================================
+ Hits         6679     6681       +2     
- Misses       1472     1476       +4     
  Partials      363      363              

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