Add BlockedOneTo as the axis type for a BlockedArray

[jishnub]

After this, the axes of a BlockArray are BlockedOneTos instead of BlockedUnitRanges:

julia> B = BlockArray([1:3;], [2,1]);

julia> axes(B,1)
2-blocked 3-element BlockedOneTo{Vector{Int64}}:
 1
 2
 ─
 3

blockedrange returns a BlockedOneTo by default:

julia> b = blockedrange(1:2)
2-blocked 3-element BlockedOneTo{ArrayLayouts.RangeCumsum{Int64, UnitRange{Int64}}}:
 1
 ─
 2
 3

julia> b = blockedrange(3, 1:2) # can obtain a `BlockedUnitRange` by specifying the first value
2-blocked 3-element BlockedUnitRange{Vector{Int64}}:
 3
 ─
 4
 5

BlockedOneTo are their own axes:

julia> b = blockedrange([2,2,3]);

julia> axes(b,1) === b
true

blocklengths may avoid allocating if the block sizes are given by an OrdinalRange:

julia> b = blockedrange(1:2);

julia> blocklengths(b)
1:2

Breaking change: this PR disallows offset vectors as arguments to blockedrange, which used to be supported earlier. Supporting this correctly is a lot of work, and having the blocklasts have offset axes adds little value to the use case. Typically, we only care about the values of blocklasts, and not their axes.

[codecov]

Codecov Report

Attention: Patch coverage is 98.09524% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 95.35%. Comparing base (7766ac9) to head (7c02524).

Files	Patch %	Lines
src/blockaxis.jl	98.30%	1 Missing ⚠️
src/blockcholesky.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-1.0     #348      +/-   ##
===============================================
+ Coverage        95.33%   95.35%   +0.01%     
===============================================
  Files               16       16              
  Lines             1500     1527      +27     
===============================================
+ Hits              1430     1456      +26     
- Misses              70       71       +1     

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[putianyi889]

Does it mean the axes of blockarrays have to start from 1, e.g. offset not allowed?

[jishnub]

Yes, this PR disallows this. Is there a use case for block arrays with offset axes? One may wrap it in an OffsetArray if indexing is the concern.

[putianyi889]

I'm not particularly concerned about this. It'll simplify many other implementations.