Added useful functions to so2

[tgurriet]

No description provided.

[codecov-commenter]

Codecov Report

Merging #45 (31837af) into master (ea1da4b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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@@           Coverage Diff           @@
##           master      #45   +/-   ##
=======================================
  Coverage   99.53%   99.54%           
=======================================
  Files          49       49           
  Lines        2606     2618   +12     
=======================================
+ Hits         2594     2606   +12     
  Misses         12       12           

Files	Coverage Δ
include/smooth/so2.hpp	100.00% <100.00%> (ø)

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