BUG: suppress RuntimeWarning in snow.categorize and outliers.zscore

[Omesh37]

Closes #237

Description

Two functions emitted RuntimeWarning for valid edge-case inputs:
1. snow.py:229 —> invalid value encountered in divide when modeled_voltage_with_snow_copy contains zeros. The existing np.errstate(divide='ignore') did not cover the invalid case (0/0). Added invalid='ignore' to the existing errstate.

2. outliers.py:78 —> Precision loss RuntimeWarning from scipy.stats.zscore when all input values are identical. Wrapped the zscore call with np.errstate(invalid='ignore').

• Closes BUG: RuntimeWarning in snow.categorize and outliers.zscore for edge case inputs #237
• Added tests to cover all new or modified code.
• Non-API functions clearly documented with docstrings or comments as necessary.
• Adds description and name entries in the appropriate "what's new" file
  in docs/whatsnew
  for all changes. Includes link to the GitHub Issue with :issue:`num`
  or this Pull Request with :pull:`num`. Includes contributor name
  and/or GitHub username (link with :ghuser:`user`).
• Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels and Milestone are assigned to the Pull Request and linked Issue.