When creating vectors of GMTdataset, assign them a data type as well

[joa-quim]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.77465% with 3 lines in your changes missing coverage. Please review.

Project coverage is 86.05%. Comparing base (86a6247) to head (df0b04a).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/utils_types.jl	83.33%	2 Missing ⚠️
src/grdinterpolate.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1685      +/-   ##
==========================================
+ Coverage   85.96%   86.05%   +0.09%     
==========================================
  Files         162      162              
  Lines       22762    22760       -2     
==========================================
+ Hits        19567    19586      +19     
+ Misses       3195     3174      -21     

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