Add docstrings to LowRankFun and PartialInverseOperator

[jishnub]

Also, limit the bandwidth of a PartialInverseOperator to Tuple{Any, Any}.

[codecov]

Codecov Report

Base: 71.14% // Head: 67.33% // Decreases project coverage by -3.80% ⚠️

Coverage data is based on head (3519e1a) compared to base (d26f787).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
- Coverage   71.14%   67.33%   -3.81%     
==========================================
  Files          77       77              
  Lines        8015     8018       +3     
==========================================
- Hits         5702     5399     -303     
- Misses       2313     2619     +306     

Impacted Files	Coverage Δ
src/Multivariate/LowRankFun.jl	67.57% <ø> (+1.56%)	⬆️
src/Operators/general/PartialInverseOperator.jl	98.21% <100.00%> (+1.78%)	⬆️
src/onehotvector.jl	100.00% <100.00%> (ø)
src/Operators/almostbanded/LowRankOperator.jl	0.00% <0.00%> (-83.34%)	⬇️
src/Operators/almostbanded/LowRankPertOperator.jl	0.00% <0.00%> (-63.34%)	⬇️
src/Caching/matrix.jl	42.39% <0.00%> (-27.18%)	⬇️
src/Operators/functionals/CalculusFunctional.jl	60.60% <0.00%> (-18.19%)	⬇️
src/Multivariate/VectorFun.jl	58.33% <0.00%> (-14.59%)	⬇️
src/Spaces/Spaces.jl	51.25% <0.00%> (-14.19%)	⬇️
src/Caching/almostbanded.jl	83.85% <0.00%> (-14.04%)	⬇️
... and 31 more

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