Add <, >, <= and >= GMTgrid operations.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 90.08%. Comparing base (525951c) to head (a0b4ae1).

Files	Patch %	Lines
src/common_options.jl	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1400 +/- ## ========================================== - Coverage 90.89% 90.08% -0.81% ========================================== Files 134 134 Lines 17260 17275 +15 ========================================== - Hits 15688 15562 -126 - Misses 1572 1713 +141 ```

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