lowess: allow missing input_core_dim

[mathause]

• Closes #xxx
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

Allow missing core dims in mesmer.stats.lowess. This will help to pass arbitrary Datasets.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05% 🎉

Comparison is base (e38fef0) 86.33% compared to head (694c5f2) 86.39%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #283      +/-   ##
==========================================
+ Coverage   86.33%   86.39%   +0.05%     
==========================================
  Files          38       38              
  Lines        1537     1543       +6     
==========================================
+ Hits         1327     1333       +6     
  Misses        210      210              

Flag	Coverage Δ
unittests	86.39% <100.00%> (+0.05%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
mesmer/stats/smoothing.py	95.23% <100.00%> (+1.90%)	⬆️

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