only tribuo models can deal with "string" target columns

[behrica]

I open this here as a meta-issue
The fixes need to happen in the various model plugins.

 ;; only tribuo can deal with "string" target
 (concat
  ;other-specs
  ;xgboost-specs
  tribuo-model-specs
  ;smile-model-specs
  ;sklearn-model-specs
  )

We can close it when all models are fixed

[behrica]

Only tribuo can currently correctly handle "categorical target vars" in all forms:

• "strings"
• "integers" + "doubles"
• categorical map

[behrica]

several issues created:
scicloj/scicloj.ml.smile#25
scicloj/scicloj.ml.xgboost#9
scicloj/sklearn-clj#15