Split STL seasonal_period into separate seasonal_smoother and period

[eccabay]

Currently, in evalml/stl_decomposer.py at main · alteryx/evalml, the STLDecomposer mistakenly takes a value for seasonal and passes it to the seasonal parameters of the underlying STL algorithm. It seems that seasonal is actually a value for a window that does smoothing (possibly for the trend decomposition). This fixes that by splitting the seasonal_period parameter into separate seasonal_smoother (ideally a clearer name for this parameter) and period

[codecov]

Codecov Report

Merging #3896 (f5c7333) into main (e6b837a) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3896     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        346     346             
  Lines      36639   36640      +1     
=======================================
+ Hits       36509   36510      +1     
  Misses       130     130             

Impacted Files	Coverage Δ
...sts/decomposer_tests/test_polynomial_decomposer.py	100.0% <ø> (ø)
...omponents/transformers/preprocessing/decomposer.py	99.3% <100.0%> (+0.1%)	⬆️
...ransformers/preprocessing/polynomial_decomposer.py	100.0% <100.0%> (ø)
...nents/transformers/preprocessing/stl_decomposer.py	100.0% <100.0%> (ø)
...omponent_tests/decomposer_tests/test_decomposer.py	99.7% <100.0%> (ø)
...nent_tests/decomposer_tests/test_stl_decomposer.py	100.0% <100.0%> (ø)

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[chukarsten]

I suspect the behavior we want here is actually to warn off the period rather than the seasonal_smoother parameter here.