Convert to OffsetArrays

[jishnub]

Fixes #171 partly

Now conversion works if the OffsetArray type is fully specified (doesn't work at present if it's left as a UnionAll works now).

julia> d = Diagonal([1,1,1])
3×3 Diagonal{Int64, Vector{Int64}}:
 1  ⋅  ⋅
 ⋅  1  ⋅
 ⋅  ⋅  1

julia> convert(OffsetMatrix{Float64, Matrix{Float64}}, d)
3×3 OffsetArray(::Matrix{Float64}, 1:3, 1:3) with eltype Float64 with indices 1:3×1:3:
 1.0  0.0  0.0
 0.0  1.0  0.0
 0.0  0.0  1.0

julia> convert(OffsetMatrix{Float32, Matrix{Float32}}, d)
3×3 OffsetArray(::Matrix{Float32}, 1:3, 1:3) with eltype Float32 with indices 1:3×1:3:
 1.0  0.0  0.0
 0.0  1.0  0.0
 0.0  0.0  1.0

julia> convert(OffsetMatrix{Float32, Matrix{Float32}}, OffsetArray(d, 2, 2))
3×3 OffsetArray(::Matrix{Float32}, 3:5, 3:5) with eltype Float32 with indices 3:5×3:5:
 1.0  0.0  0.0
 0.0  1.0  0.0
 0.0  0.0  1.0

As a consequence this matrix vector product works now (used to error earlier):

julia> a = [MMatrix{2,2}(1:4) for i = 1:2];

julia> oa = [OffsetArray(ai, 0, 0) for ai in a];

julia> b = ones(2,2);

julia> b * oa
2-element Vector{OffsetMatrix{Float64, SizedMatrix{2, 2, Float64, 2, Matrix{Float64}}}}:
 [2.0 6.0; 4.0 8.0]
 [2.0 6.0; 4.0 8.0]

These constructors create copies if necessary.

I have not added the other UnionAll constructors deliberately as it'll not be clear if the OffsetArray(::AbstractArray) constructor creates a copy or not. Currently it doesn't create a copy for OffsetArray(::Array{T,0}), so creating a copy for other arrays might be confusing. On the other hand, we may choose to not create a copy at all, which will make the OffsetArray constructor behave differently from Array.

[codecov]

Codecov Report

Merging #230 (7293ff6) into master (cca160c) will increase coverage by 0.38%.
The diff coverage is 97.14%.

@@            Coverage Diff             @@
##           master     #230      +/-   ##
==========================================
+ Coverage   94.69%   95.07%   +0.38%     
==========================================
  Files           5        5              
  Lines         358      386      +28     
==========================================
+ Hits          339      367      +28     
  Misses         19       19              

Impacted Files	Coverage Δ
src/OffsetArrays.jl	98.12% <96.87%> (-0.22%)	⬇️
src/utils.jl	100.00% <100.00%> (+3.03%)	⬆️

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[johnnychen94]

it'll not be clear if the OffsetArray(::AbstractArray) constructor creates a copy or not

We won't do deep copy; most constructors don't do it. Since offsets is a tuple, copy the wrapper type or not doesn't make much difference. My point is: OffsetArray(a::OffsetArray) === OffsetArray(a::OffsetArray) is not expected to hold in general. Users' codes should not depend on these internal differences.

[jishnub]

I've added the extra constructors necessary, so now general conversions work.

julia> convert(OffsetVector, 1:4)
1:4 with indices 1:4

julia> convert(OffsetVector{Float32}, 1:4)
1.0f0:1.0f0:4.0f0 with indices 1:4

[johnnychen94]

Except for the small "bug", all others are good to me. (I like how you write tests 👍 )