Implicitly specify axes using colons in the constructor

[jishnub]

In case certain axes of an array are not offset, they may be implicitly specified using Colons in the constructor. The colons would be replaced by the corresponding axes of the parent array.

julia> OffsetArray(zeros(2,3), :, 4:6)
2×3 OffsetArray(::Array{Float64,2}, 1:2, 4:6) with eltype Float64 with indices 1:2×4:6:
 0.0  0.0  0.0
 0.0  0.0  0.0

This is useful in cases where the size along a non-offset dimension is not known a-priori.

[johnnychen94]

Nicely done! Probably there are some utils in Base that can be reused here, but I think this one is written pretty clearly and self-explained.

The failed test in the nightly build is unrelated so I believe this is good to get merged; the fix for the nightly build can be done in other PR.

[jishnub]

This works with offset axes as well, and is not limited to non-offset ones.

julia> OffsetArray(zeros(3:4), :)
2-element OffsetArray(::Array{Float64,1}, 3:4) with eltype Float64 with indices 3:4:
 0.0
 0.0

julia> OffsetArray(zeros(3:4, 5:6), :, 4:5)
2×2 OffsetArray(::Array{Float64,2}, 3:4, 4:5) with eltype Float64 with indices 3:4×4:5:
 0.0  0.0
 0.0  0.0

I wonder if I should add tests for these?

[johnnychen94]

yes, we may want to make sure the axes is not changed with : as a regression test.

[johnnychen94]

reopen this PR to retrigger the nightly test based on #140

[codecov]

Codecov Report

Merging #139 into master will increase coverage by 4.16%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #139      +/-   ##
==========================================
+ Coverage   86.29%   90.45%   +4.16%     
==========================================
  Files           2        2              
  Lines         197      220      +23     
==========================================
+ Hits          170      199      +29     
+ Misses         27       21       -6     

Impacted Files	Coverage Δ
src/OffsetArrays.jl	90.34% <100.00%> (+3.80%)	⬆️
src/axes.jl	90.90% <0.00%> (+5.54%)	⬆️

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[johnnychen94]

@jishnub If it's okay with you to wait for a few more days, I'd like to make a new release by this weekend with a patch for #136, too.

[jishnub]

I don't have issues with the delay, but I'm just wondering if this should go in a minor release whereas the bugfix for #136 should go in a patch release? Perhaps I'm not clear about the release sequence