ENH: Add ability to append to a model

[codecov[bot]]

Codecov Report

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

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

:exclamation: 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

```diff @@ 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](https://app.codecov.io/gh/bashtage/arch/pull/674?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kevin+Sheppard) | Coverage Δ | | |---|---|---| | [arch/utility/array.py](https://app.codecov.io/gh/bashtage/arch/pull/674?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kevin+Sheppard#diff-YXJjaC91dGlsaXR5L2FycmF5LnB5) | `92.54% <16.66%> (-6.13%)` | :arrow_down: | | [arch/univariate/mean.py](https://app.codecov.io/gh/bashtage/arch/pull/674?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kevin+Sheppard#diff-YXJjaC91bml2YXJpYXRlL21lYW4ucHk=) | `96.57% <17.64%> (-1.89%)` | :arrow_down: | | [arch/univariate/base.py](https://app.codecov.io/gh/bashtage/arch/pull/674?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Kevin+Sheppard#diff-YXJjaC91bml2YXJpYXRlL2Jhc2UucHk=) | `98.17% <25.00%> (-0.78%)` | :arrow_down: |

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