Add a gmt_centroid_area() fun to directly ccall the GMT C lib function.

[joa-quim]

No description provided.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 73.68421% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.19%. Comparing base (98bec96) to head (19dd27e).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/psxy.jl	0.00%	4 Missing ⚠️
src/libgmt.jl	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1785      +/-   ##
==========================================
- Coverage   86.21%   86.19%   -0.02%     
==========================================
  Files         167      166       -1     
  Lines       24257    24207      -50     
==========================================
- Hits        20912    20865      -47     
+ Misses       3345     3342       -3     

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