Automatically set the GDAL_DATA env variable

[joa-quim]

Some code reorganization (move functions between files).

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 90.16%. Comparing base (1805b62) to head (6174599).

Files	Patch %	Lines
src/gmtreadwrite.jl	42.85%	4 Missing ⚠️
src/gdal_tools.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1390      +/-   ##
==========================================
+ Coverage   90.12%   90.16%   +0.03%     
==========================================
  Files         134      134              
  Lines       17171    17172       +1     
==========================================
+ Hits        15476    15483       +7     
+ Misses       1695     1689       -6     

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