Add support for knn enhanced ar(x) forecastors

[lzlzlizi]

Hi, @dajmcdon. Those commits add the support for KNN enhanced iterative, dynamic iterative, and direct methods. The direct method support the arx mode while the two iterative methods only support the ar mode.

A few comments:

1. The search-train-predict-residual procedures are tangled together because we need the new_data first to start. Although we can technically split some of them apart, splitting will induce non-trivial space or computation redundancy.
2. arx_forecastor.R was accidentally changed when I apply auto formating for the scripts. Those changes can be revoked if needed.
3. KNN searching is supported by package tensr. Do we need to manually add it to dependencies?

[lzlzlizi]

Sorry for bombarding you with the emails of R-CMD-check failures. Didn't realize it will send emails to everyone. I will temporarily close the PR until I fixed the check failure issue.