add eachobs, eachbatch, kfolds, leavepout

[CarloLucibello]

Changes:

• the implementation of eachobs and eachbatch is based on generators and doesn't rely on BufferGetObs (which I didn't port)
• eachobs doesn't rely on ObsView
• eachobs and eachbatch have a new buffer keyword arg
• leaveout is renamed to leavepout
• kfolds and leavepout return generators instead of FoldView (which I didn't port).

Generally, I'm trying to go in the direction of #8 and have much less container/view types

[codecov-commenter]

Codecov Report

Merging #9 (2e3e9dc) into main (02e1366) will increase coverage by 3.45%.
The diff coverage is 94.28%.

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   74.54%   78.00%   +3.45%     
==========================================
  Files           7        9       +2     
  Lines         165      200      +35     
==========================================
+ Hits          123      156      +33     
- Misses         42       44       +2     

Impacted Files	Coverage Δ
src/dataview.jl	71.42% <ø> (ø)
src/folds.jl	92.00% <92.00%> (ø)
src/dataiterator.jl	100.00% <100.00%> (ø)

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