Relax orthogonal

[AlexRobson]

One issue with this I've found recently revisiting orthogonal is that it restricts wrapper arrays. As an example, present internal pipelines use AxisKeys and NamedDims - however the StridedArray restriction prevents this:

using ParameterHandling: value, flatten

A = NamedDimsArray{(:id, :_)}(rand(5,5));

orthogonal(A)
ERROR: MethodError: no method matching orthogonal(::NamedDimsArray{(:id, :_), Float64, 2, Matrix{Float64}})
Closest candidates are:
  orthogonal(::StridedMatrix{var"#s3"} where var"#s3"<:Real) at /Users/alexrobson/.julia/packages/ParameterHandling/AGJx1/src/parameters.jl:181

Rewriting orthgonal to allow for abstractmatrix allows this to work:

using NamedDims, ParameterHandling

A = NamedDimsArray{(:id, :_)}(rand(5,5));

v, unflatten = flatten(orthogonal(A));

unflatten(v) # ParameterHandling.Orthogonal{NamedDimsArray{(:id, :_), Float64, 2, Matrix{Float64}}

With a nod to #22 AFAICT only flatten needs to be specialised?

[codecov[bot]]

Codecov Report

Merging #53 (6a65c56) into master (0d59a83) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #53   +/-   ##
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  Coverage   96.80%   96.80%           
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  Files           7        7           
  Lines         188      188           
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  Hits          182      182           
  Misses          6        6           

Impacted Files	Coverage Δ
src/parameters_matrix.jl	100.00% <100.00%> (ø)

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[AlexRobson]

I didn't include any test changes here as it's just a relaxation, would add a test-dep, and covered by existing. But the example code in the description can easily be converted into them.

[rofinn]

Yeah, based on what the functions are doing I don't see a strong reason to restrict the type. I'd appreciate @willtebbutt's input on this though given #22. Don't forget to update the docstrings.