filter: Move register_arguments to top of file

[victorlin]

Description of proposed changes

Having register_arguments at the top of the file makes more sense to me
since it describes the high-level functionality of this module.

Related issue(s)

N/A

Testing

N/A

[codecov]

Codecov Report

Merging #995 (7dfd547) into master (a1d418d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #995   +/-   ##
=======================================
  Coverage   59.24%   59.24%           
=======================================
  Files          50       50           
  Lines        6266     6266           
  Branches     1588     1588           
=======================================
  Hits         3712     3712           
  Misses       2292     2292           
  Partials      262      262           

Impacted Files	Coverage Δ
augur/__version__.py	100.00% <100.00%> (ø)
augur/filter.py	95.78% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2dd2d4c...7dfd547. Read the comment docs.