Various Improvments

[gragusa]

1. Accommodate AbstractArrays
2. Make type persistent (e.g., if the original matrix passed to lrvar is Float32 return a Float32)
3. Various cosmetic improvements

[codecov-commenter]

Codecov Report

Merging #59 (c36216e) into master (9ecb322) will increase coverage by 1.81%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   73.79%   75.60%   +1.81%     
==========================================
  Files          10       10              
  Lines         828      787      -41     
==========================================
- Hits          611      595      -16     
+ Misses        217      192      -25     

Impacted Files	Coverage Δ
src/CovarianceMatrices.jl	100.00% <ø> (ø)
src/types.jl	74.19% <ø> (-3.94%)	⬇️
src/lrvar.jl	64.86% <77.27%> (-2.30%)	⬇️
src/HAC.jl	90.22% <80.76%> (+4.91%)	⬆️
src/CovarianceMatrix.jl	97.87% <100.00%> (+18.28%)	⬆️
src/glm.jl	89.04% <100.00%> (-0.23%)	⬇️
src/interface_api.jl	50.00% <100.00%> (ø)
... and 5 more

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