Add types in function signatures.

[joa-quim]

No description provided.

[codecov-io]

Codecov Report

Merging #299 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #299   +/-   ##
=======================================
  Coverage   93.39%   93.39%           
=======================================
  Files          80       80           
  Lines        3665     3665           
=======================================
  Hits         3423     3423           
  Misses        242      242

Impacted Files	Coverage Δ
src/gmt_main.jl	78.65% <ø> (ø)	⬆️
src/grdimage.jl	100% <ø> (ø)	⬆️
src/common_options.jl	95.9% <ø> (ø)	⬆️

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