ENH: Add ability to append to a model

[bashtage]

No description provided.

[codecov]

Codecov Report

Patch coverage: 18.91% and project coverage change: -0.20 ⚠️

Comparison is base (98a2bfa) 99.43% compared to head (478d0f6) 99.24%.

❗ Current head 478d0f6 differs from pull request most recent head 4cd013d. Consider uploading reports for the commit 4cd013d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #674      +/-   ##
==========================================
- Coverage   99.43%   99.24%   -0.20%     
==========================================
  Files          74       74              
  Lines       15509    15544      +35     
  Branches     2086     2095       +9     
==========================================
+ Hits        15422    15427       +5     
- Misses         63       93      +30     
  Partials       24       24              

Impacted Files	Coverage Δ
arch/utility/array.py	92.54% <16.66%> (-6.13%)	⬇️
arch/univariate/mean.py	96.57% <17.64%> (-1.89%)	⬇️
arch/univariate/base.py	98.17% <25.00%> (-0.78%)	⬇️

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