delete multiple inclusion of same file

[OkonSamuel]

This PR

1. deletes multiple inclusion of some files in at MLJTuning/src/MLJTuning.jl
2. changes type annotation of next field in ExplicitState struct. i.e

   
   struct ExplicitState{R,N}
   range::R # a model-generating iterator
   next::Union{Nothing, N} # to hold output of `iterate(range)`
   end

ExplicitState(r::R, ::Nothing) where R = ExplicitState{R,Nothing}(r,nothing) ExplictState(r::R, n::N) where {R,N} = ExplicitState{R,Union{Nothing,N}}(r,n)

was replaced by
```julia
struct ExplicitState{R, N}
    range::R # a model-generating iterator
    next::N # to hold output of `iterate(range)`
end

ExplictState(r::R, n::N) where {R,N} = ExplicitState{R, Union{Nothing, N}}(r, n)

This was done becase ExplicitState is an immutable struct and Union{Nothing, Nothing}==Nothing

@ablaom. You could merge this and tag a new release if your satisfied with the second change.

[ablaom]

Awesome. Thanks for diagnosing this @OkonSamuel