Append to DNDSparse to parse and write in chunks

[gkappler]

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I required to append to JuliaDB.DNDSparse, similar to other users (https://discourse.julialang.org/t/append-rows-to-a-juliadb-table-on-disk/44071).

To achieve this, I refactored code regarding fromchunks, vcat chunks and serialize the index out of function _loadtable into function _merge! to make appending available.

If this PR is interesting, merge! dispatch for DIndexedTable would be useful too (not yet, because I currently do not use it).