ENH: Improve Variance Ratio

[bashtage]

Add user protections in Variance Ratio

[codecov]

Codecov Report

Merging #375 into master will increase coverage by 0.00%.
The diff coverage is 98.70%.

@@           Coverage Diff           @@
##           master     #375   +/-   ##
=======================================
  Coverage   99.42%   99.43%           
=======================================
  Files          66       66           
  Lines       13171    13205   +34     
  Branches     1121     1124    +3     
=======================================
+ Hits        13095    13130   +35     
  Misses         24       24           
+ Partials       52       51    -1     

Impacted Files	Coverage Δ
arch/bootstrap/base.py	98.01% <0.00%> (ø)
arch/unitroot/_phillips_ouliaris.py	98.79% <ø> (ø)
arch/utility/cov.py	100.00% <ø> (ø)
arch/tests/unitroot/test_unitroot.py	100.00% <100.00%> (ø)
arch/unitroot/unitroot.py	99.13% <100.00%> (+0.14%)	⬆️
arch/univariate/mean.py	98.68% <100.00%> (ø)

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[coveralls]

Coverage increased (+0.0008%) to 99.686% when pulling 44dc7b4 on vr-fix-v2 into 98e8761 on master.