Add facility for adding a data front-end to a model implementation

[ablaom]

A PR to support JuliaAI/MLJBase.jl#492.

Adds:

• reformat method/fallback
• selectrows(model::Model, I, args...) -> data fallback

From the new doc-strings (edited):

---

MLJModelInterface.reformat(model, args...) -> data

Models optionally overload reformat to define transformations of
user-supplied data into some model-specific representation (e.g., from
a table to a matrix). When implemented, the MLJ user can avoid
repeating such transformations unnecessarily, and can additionally
make use of more efficient row subsampling, which is then based on the
model-specific representation of data, rather than the
user-representation. When reformat is overloaded,
selectrows(::Model, ...) must be as well (see
selectrows). Furthermore, the model fit method(s), and
operations, such as predict and transform, must be refactored to
act on the model-specific representions of the data.

To implement the reformat data front-end for a model, refer to
"Implementing a data front-end" in the MLJ
manual.

selectrows(::Model, I, data...) -> sampled_data

A model overloads selectrows whenever it buys into the optional
reformat front-end for data preprocessing. See reformat
for details. The fallback assumes data is a tuple and calls
selectrows(X, I) for each X in data, returning the results in a
new tuple of the same length. This call makes sense when X is a
table, abstract vector or abstract matrix. In the last two cases, a
new object and not a view is returned.

[codecov-io]

Codecov Report

Merging #76 (9c937d8) into dev (2c358e9) will decrease coverage by 0.46%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##              dev      #76      +/-   ##
==========================================
- Coverage   99.04%   98.57%   -0.47%     
==========================================
  Files           9        9              
  Lines         209      211       +2     
==========================================
+ Hits          207      208       +1     
- Misses          2        3       +1     

Impacted Files	Coverage Δ
src/MLJModelInterface.jl	100.00% <ø> (ø)
src/model_api.jl	83.33% <83.33%> (-16.67%)	⬇️

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