joinpartitions should take a promote kwarg

[rofinn]

To improve usability of the joinpartitions function I think we should probably support a promote::Bool kwarg which will allow us to widen the ChainedVector eltypes as we go. This is something DataFrames.jl already supports with its append! implementation.

https://github.com/JuliaData/DataFrames.jl/blob/f690aa49e958f51e0c3c579b6def1f11be214d98/src/dataframe/dataframe.jl#L1301

Example error which is annoying to work around is.

ArgumentError: string too large (36) to convert to InlineStrings.String31
Stacktrace:
[1] stringtoolong(T::Type, n::Int64)
@ InlineStrings ~/.julia/packages/InlineStrings/2HAHk/src/InlineStrings.jl:186
[2] InlineString
@ ~/.julia/packages/InlineStrings/2HAHk/src/InlineStrings.jl:209 [inlined]
[3] convert
@ ./strings/basic.jl:232 [inlined]
[4] convert
@ ./missing.jl:69 [inlined]
[5] getpoolidx
@ ~/.julia/packages/PooledArrays/jKDrZ/src/PooledArrays.jl:548 [inlined]
[6] push!(pv::PooledArrays.PooledVector{Union{Missing, InlineStrings.String31}, UInt32, Vector{UInt32}}, v::InlineStrings.String63)
@ PooledArrays ~/.julia/packages/PooledArrays/jKDrZ/src/PooledArrays.jl:594
[7] push!
@ ~/.julia/packages/SentinelArrays/VbnBp/src/chainedvector.jl:451 [inlined]
[8] append!(A::SentinelArrays.ChainedVector{Union{Missing, InlineStrings.String31}, PooledArrays.PooledVector{Union{Missing, InlineStrings.String31}, UInt32, Vector{UInt32}}}, B::PooledArrays.PooledVector{Union{Missing, InlineStrings.String63}, UInt32, Vector{UInt32}})
@ SentinelArrays ~/.julia/packages/SentinelArrays/VbnBp/src/chainedvector.jl:608
[9] (::TableOperations.var"#23#26"{Vector{SentinelArrays.ChainedVector}, DataFrames.DataFrameColumns{DataFrames.DataFrame}})(i::Int64)
@ TableOperations ~/.julia/packages/TableOperations/ErT2o/src/TableOperations.jl:312
[10] foreach(f::TableOperations.var"#23#26"{Vector{SentinelArrays.ChainedVector}, DataFrames.DataFrameColumns{DataFrames.DataFrame}}, itr::UnitRange{Int64})
@ Base ./abstractarray.jl:2141
[11] joinpartitions(x::Tables.Partitioner{Base.Generator{Vector{String}, Tables.var"#9#10"{S3DB.var"#220#221"}}})