Horizon as list

[deatinor]

What does this implement/fix? Explain your changes.

It adds the possibility of having an horizon_shift as list, instead of just range.

[codecov]

Codecov Report

Merging #164 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #164   +/-   ##
=======================================
  Coverage   95.92%   95.93%           
=======================================
  Files          43       43           
  Lines        1351     1354    +3     
  Branches      115      115           
=======================================
+ Hits         1296     1299    +3     
  Misses         39       39           
  Partials       16       16           

Impacted Files	Coverage Δ
gtime/model_selection/horizon_shift.py	100.00% <100.00%> (ø)
gtime/time_series_models/ar.py	100.00% <100.00%> (ø)
gtime/time_series_models/base.py	100.00% <100.00%> (ø)

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