constructor for MixedModel dispatches to subtype

[palday]

As previously discussed (since users can already just specify write fit(MixedModel, ....)

[codecov]

Codecov Report

Merging #572 (04b7003) into main (694faa1) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 04b7003 differs from pull request most recent head 99bd31a. Consider uploading reports for the commit 99bd31a to get more accurate results

@@           Coverage Diff           @@
##             main     #572   +/-   ##
=======================================
  Coverage   96.24%   96.25%           
=======================================
  Files          28       28           
  Lines        2503     2509    +6     
=======================================
+ Hits         2409     2415    +6     
  Misses         94       94           

Impacted Files	Coverage Δ
src/mixedmodel.jl	92.59% <100.00%> (+0.92%)	⬆️

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