Enable longoptions support for grdfft.

[rbdavis]

Description of proposed changes

Python differences:

No Python module.

Julia differences:

Changed -E radial_power to power_spectrum. Spectrum can be x or y
as well as radial.

Fixes #

Reminders

• Make sure that your code follows our style. Use the other functions/files as a basis.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.

[joa-quim]

This PR has covers two cases: grdfft and grdedit.

And regarding grdfft, it happening again. When you replace a GMT.jl long option that is unique (the -E case) that breaks names intersection with GMT.jl. Besides that, I prefer radial_power (rare to find in other softs) to power_pectrum.

[joa-quim]

Same comment as in the other PR. This breaks with GMT.jl

[rbdavis]

This PR has covers two cases: grdfft and grdedit.

My apologies to all, but something strange happened this morning while I was uploading the various new longopts-* branches. There was a problem with the longopts-grdedit upload, and because of this all of the subsequent uploads and PRs I did were impacted and you will be seeing a grdeit commit notice in each one. Too long to explain here in any coherent way (and in fact I do not understand exactly what's going on), but in all of these other PRs you should see only one changed file -- in the case of this PR, that's grdfft_inc.h. You should ignore the commit notices at the top of these PRs which refer to grdedit, as that was just me putting grdedit_inc.h back to its original unedited form.