BUG: Correct 2d model initializatiion in HAR

[bashtage]

Clarify how 2D is used and add test

closes #411

[codecov]

Codecov Report

Merging #417 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #417   +/-   ##
=======================================
  Coverage   99.36%   99.36%           
=======================================
  Files          72       72           
  Lines       13308    13317    +9     
  Branches     1131     1131           
=======================================
+ Hits        13223    13232    +9     
  Misses         29       29           
  Partials       56       56           

Impacted Files	Coverage Δ
arch/tests/univariate/test_mean.py	99.76% <100.00%> (+<0.01%)	⬆️
arch/univariate/mean.py	98.34% <100.00%> (ø)

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[coveralls]

Coverage increased (+0.003%) to 99.781% when pulling 20879d4 on har-docs into a8d3c0c on master.