Add narrowtypes functionality

[quinnj]

@anandijain, I'm not able to push to your branch, but I made some changes and rebased your changes here; take a look if it seems ok to you. Then we can finish your todos by adding docs/tests.

[codecov[bot]]

Codecov Report

Merging #14 (f4c4d50) into master (b3703ba) will decrease coverage by 2.04%. The diff coverage is 62.50%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
- Coverage   90.73%   88.68%   -2.05%     
==========================================
  Files           1        1              
  Lines         205      221      +16     
==========================================
+ Hits          186      196      +10     
- Misses         19       25       +6     

Impacted Files	Coverage Δ
src/TableOperations.jl	88.68% <62.50%> (-2.05%)	:arrow_down:

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[anandijain]

Thanks a lot for the help! These changes all look good.

I just added some tests, which I'll push now. I based them off of other testsets.

Many of the other lazy structs have functions that return an anonymous function ie x->filter(f, x), is this needed?

[anandijain]

Actually, I'm able to push to my fork, but I think since I don't have juliadata/ write access, I won't be able to add commits (is that right?)

Since it's so few lines, here are the tests:

ctable_type_any = (A=Any[1, missing, 3], B=Any[1.0, 2.0, 3.0], C=Any["hey", "there", "sailor"])

@testset "TableOperations.narrowtypes" begin

nt = TableOperations.narrowtypes(ctable_type_any)
@test Tables.istable(nt)
@test Tables.columnaccess(nt)
@test Tables.schema(nt) == Tables.schema(ctable)
@test Tables.columnnames(nt) == Tables.columnnames(ctable)

end

[quinnj]

Thanks for the contribution @anandijain!

[anandijain]

Thanks for the help!