Start work on TableOperations.joinpartitions

[quinnj]

With the new Tables.partitions functionality in the Tables.jl package, it can be helpful for non-partitioned in-memory table structures to have a way to treat the separate partitions as a single, long partition. The SentinelArrays.jl package includes a ChainedVector type that allows lazily "chaining" AbstractArrays together, treated as a single, long vector. TableOperations.joinpartitions(x) takes any partitioned input x, and appends the columns together via ChainedVector, producing a JoinedPartitions object which itself, satisfies the Tables.columns interface, i.e. you could do df = DataFrame(TableOperations.joinpartitions(x)) to concatenate all partitions of the input and operate on them as a whole in a DataFrame.

Still need to add tests and docs.

[codecov[bot]]

Codecov Report

Merging #10 into master will decrease coverage by 0.26%. The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
- Coverage   94.26%   94.00%   -0.27%     
==========================================
  Files           1        1              
  Lines         122      150      +28     
==========================================
+ Hits          115      141      +26     
- Misses          7        9       +2     

Impacted Files	Coverage Δ
src/TableOperations.jl	94.00% <90.47%> (-0.27%)	:arrow_down:

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