Add DoubleTurn unit

[waldyrious]

This is a re-submission of #9, including the changes discussed there. All credit goes to @JeffreySarnoff. Fixes #8.

[codecov-io]

Codecov Report

Merging #11 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master    #11   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           1      1           
  Lines          13     13           
=====================================
  Hits           13     13

Impacted Files	Coverage Δ
src/UnitfulAngles.jl	100% <ø> (ø)	⬆️

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[coveralls]

Coverage remained the same at 100.0% when pulling 6203a60 on waldyrious:double-turn into c3dc5cd on yakir12:master.

[yakir12]

Thanks!

[coveralls]

Coverage remained the same at 100.0% when pulling 8e1dda3 on waldyrious:double-turn into c3dc5cd on yakir12:master.