Update the model registry

[ablaom]

Besides updating metadata for all models, this adds:

• Maxnet = ["MaxnetBinaryClassifier"]
• SymbolicRegression = ["SRTestRegressor", "MultitargetSRTestRegressor"]
• MLJTransforms = ["EntityEmbedder"]

It also removes the models from PartialLeastSquaresRegressor.jl

closes #572, #561

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.52%. Comparing base (c6f536f) to head (fc66203).
Report is 3 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #581   +/-   ##
=======================================
  Coverage   75.52%   75.52%           
=======================================
  Files          17       17           
  Lines        1152     1152           
=======================================
  Hits          870      870           
  Misses        282      282           

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