Make main as the base branch

[zhiqwang]

We rename master to main in this PR.

[codecov]

Codecov Report

Merging #200 (e0ce161) into main (4b3454e) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #200   +/-   ##
=======================================
  Coverage   96.83%   96.83%           
=======================================
  Files          10       10           
  Lines         601      601           
=======================================
  Hits          582      582           
  Misses         19       19           

Flag	Coverage Δ
unittests	96.83% <ø> (ø)

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