add Baxter King filter#223

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Shunsuke-Hori wants to merge 2 commits into

QuantEcon:masterfrom

Shunsuke-Hori:add-bk_filter

Commits on Oct 8, 2018

add bk filter and its test
Shunsuke-Hori
committed

Commits on Sep 26, 2020

fix grammer error
Shunsuke-Hori
committed