Add virtual properties for values and indices

[mkitti]

This is a full implementation of #316. Virtual properties for values and indices are added and documented.

This differs from the proposed implementation in #316 in that the properties return IdOffsetArray rather than UnitRanges.

Another discovered benefit is the ability to forward the properties to construct another IdOffsetRange.

        _values = 5:8
        _indices = 6:9
        id = IdOffsetRange(values=_values, indices=_indices)
        @test id.values === values(id)
        @test id.values == _values
        @test id.indices === eachindex(id)
        @test id.indices == _indices
        id2 = IdOffsetRange(; id.values, id.indices)
        @test id == id2
        id3 = IdOffsetRange(; id.indices, id.values)
        @test id == id3

[codecov[bot]]

Codecov Report

Merging #318 (2945d61) into master (08dc371) will decrease coverage by 1.29%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #318      +/-   ##
==========================================
- Coverage   96.45%   95.16%   -1.30%     
==========================================
  Files           5        5              
  Lines         451      434      -17     
==========================================
- Hits          435      413      -22     
- Misses         16       21       +5     

Impacted Files	Coverage Δ
src/axes.jl	97.61% <50.00%> (-2.39%)	:arrow_down:
src/utils.jl	96.00% <0.00%> (-2.00%)	:arrow_down:
src/OffsetArrays.jl	97.50% <0.00%> (-0.80%)	:arrow_down:

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[jishnub]

Looking at the example from #316,

julia> Base.@kwdef struct MyIdOffsetRange
                  values::UnitRange
                  indices::UnitRange
              end
MyIdOffsetRange

julia> x = MyIdOffsetRange(values=5:8, indices=5:8)
MyIdOffsetRange(5:8, 5:8)

julia> x.values
5:8

julia> x.values === x
false

In this example, the returned results correspond exactly to the displayed ones, as both directly read the fields of the struct. However, for an IdOffsetRange, we obtain another IdOffsetRange, which isn't what was displayed.

julia> id
OffsetArrays.IdOffsetRange(values=5:8, indices=6:9)

julia> id.values
OffsetArrays.IdOffsetRange(values=5:8, indices=6:9)

julia> id.values === id
true

This isn't an objection to the PR, but I wonder if we need to be consistent here?

[mkitti]

The consistency choice I made here was for

id.values === values(id)

values is part of the API already.

If you really wanted a UnitRange, this could work:

UnitRange(id.values)

For that reason, I added it to the documentation.