numobs and getobs

[CarloLucibello]

Porting nobs, getobs and getobs! from JuliaML/LearnBase.jl#53 with the following differences

• nobs is renamed to numobs, to avoid conflicts with StatsBase.nobs
• We get rid of the obsdim keyword, which causes a lot of additional complexity in the implementation here and in downstream packages. Wrapper types can be used in place of it.

[darsnack]

What would the wrapper type alternatives to the keyword look like?

[CarloLucibello]

something like this for obsdim=1

struct MyDataset
  A::AbstractArray
end
getobs(d::MyDataset, i) = d[i, :]

We can also provide a ArrayDataset{D} where D parameterizes the observation dimension. I think it is better to have the type set the observation dim than having to deal with the obsdim keyword during the whole downstream pipeline

[CarloLucibello]

Merging as I would like to go on with the port

[ToucheSir]

If/when JuliaLang/julia#32310 lands, that could be used over a custom dataset type.