Introduce OneElementArray

[mtfishman]

This is a sparse type that only stores a single structurally nonzero element. This is very similar to FillArrays.OneElement, but I'm reimplementing a version here for better compatibility with the sparse types we are developing, and to have more control over the operations we define. See also OneHotArrays.OneHotArray for a similar type.

A lot of algebra operations "just work" for this type, but don't yet necessarily preserve the OneElementArray type (when that is possible) and instead fall back to generic code that constructs SparseArrayDOK, for example:

julia> a = oneelement((1, 2), (2, 2))
2×2 SparseArraysBase.OneElementMatrix{Bool, Tuple{Int64, Int64}, Tuple{Base.OneTo{Int64}, Base.OneTo{Int64}}, typeof(SparseArraysBase.default_getunstoredindex)}:
 ⋅  true
 ⋅      ⋅

julia> 2a
2×2 SparseArraysBase.SparseMatrixDOK{Int64, typeof(SparseArraysBase.default_getunstoredindex)}:
 ⋅  2
 ⋅   ⋅

Note that most of the code is defining various constructors, and probably that can be simplified.

The idea is that this will serve as the basis for the next-generation oneelement function for constructing ITensors with a single element, see ITensor/ITensorBase.jl#26.

[codecov]

Codecov Report

Attention: Patch coverage is 67.88321% with 44 lines in your changes missing coverage. Please review.

Project coverage is 75.05%. Comparing base (b41609b) to head (6da5a80).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/oneelementarray.jl	67.88%	44 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #26       +/-   ##
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+ Coverage   62.54%   75.05%   +12.51%     
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  Files           6        7        +1     
  Lines         283      421      +138     
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+ Hits          177      316      +139     
+ Misses        106      105        -1     

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