Add a mean term in ARIMA

[guilhermebodin]

No description provided.

[codecov]

Codecov Report

Merging #219 (8045681) into master (20c9333) will increase coverage by 0.01%.
The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master     #219      +/-   ##
==========================================
+ Coverage   91.57%   91.59%   +0.01%     
==========================================
  Files          22       22              
  Lines        1472     1499      +27     
==========================================
+ Hits         1348     1373      +25     
- Misses        124      126       +2     

Impacted Files	Coverage Δ
src/models/locallevelexplanatory.jl	100.00% <ø> (ø)
src/prints.jl	4.76% <0.00%> (ø)
src/models/arima.jl	96.07% <87.09%> (-0.54%)	⬇️

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