`Content` types need to become fully parameterized

[Moelf]

Previous title: abstract type Content returns Any under eltype(x::Content)

I'm trying to make RecordArrays to/from_table round-tripable, and come across:
https://github.com/jpivarski/AwkwardArray.jl/blob/b53fed9b9fe99d1dc459c37bba9f70e4cd950abd/src/AwkwardArray.jl#L105

the ITEM where ITEM doesn't do anything because:

julia> ITEM where ITEM
Any

so effectively the "element type" of a Content{BEHAVIOR} is not encoded in the type parameters -- this may be fine. But I just want to make sure this is intended before I go about and define eltype() for each of the subtypes of Content

[Moelf]

basically I want to fix this:

julia> typeof(akt)
AwkwardArray.RecordArray{(:Muon_phi, :Muon_pt, :Muon_eta), Tuple{AwkwardArray.ListOffsetArray{Vector{Int64}, AwkwardArray.PrimitiveArray{Float32, Vector{Float32}, :default}, :default}, AwkwardArray.ListOffsetArray{Vector{Int64}, AwkwardArray.PrimitiveArray{Float32, Vector{Float32}, :default}, :default}, AwkwardArray.ListOffsetArray{Vector{Int64}, AwkwardArray.PrimitiveArray{Float32, Vector{Float32}, :default}, :default}}, :default}

julia> eltype(akt)
Any

fixing for only RecordArrays is simple, just define:

Base.eltype(::RecordArray{FIELDS,CONTENTS,BEHAVIOR}) where {FIELDS,CONTENTS,BEHAVIOR} = Record{FIELDS,CONTENTS,BEHAVIOR}

julia> eltype(akt) == typeof(akt[1])
true

but in case you actually wanted to have this in Content type parameter

[Moelf]

ultimately, Tables.schema(input::AwkwardArray.RecordArray) should give correct schema!

[Moelf]

this is now fixed:

julia> o = AwkwardArray.ListOffsetArray([0,2,2,3], AwkwardArray.PrimitiveArray([1,2,3,4]))
3-element AwkwardArray.ListOffsetArray{Vector{Int64}, AwkwardArray.PrimitiveArray{Int64, Vector{Int64}, :default}, :default}:
 [1, 2]
 0-element AwkwardArray.PrimitiveArray{Int64, Vector{Int64}, :default}
 [3]

julia> a = AwkwardArray.RecordArray((;x=AwkwardArray.PrimitiveArray([1,2,3]), o))
3-element AwkwardArray.RecordArray{(:x, :o), Tuple{AwkwardArray.PrimitiveArray{Int64, Vector{Int64}, :default}, AwkwardArray.ListOffsetArray{Vector{Int64}, AwkwardArray.PrimitiveArray{Int64, Vector{Int64}, :default}, :default}}, :default}:
 {x: 1, o: [1, 2]}
 {x: 2, o: []}
 {x: 3, o: [3]}

julia> eltype(a)
AwkwardArray.Record{(:x, :o), Tuple{AwkwardArray.PrimitiveArray{Int64, Vector{Int64}, :default}, AwkwardArray.ListOffsetArray{Vector{Int64}, AwkwardArray.PrimitiveArray{Int64, Vector{Int64}, :default}, :default}}, :default}