Clarify non-implemented options in sph2grd.rst

[ZMAlt]

Added notes indicating that options -D and -E are not implemented yet.

gmt/src/sph2grd.c

Lines 156 to 165 in 0fbea58

	case 'D': /* Evaluate derivative solutions */
	n_errors += gmt_M_repeated_module_option (API, Ctrl->D.active);
	n_errors += gmt_get_required_char (GMT, opt->arg, opt->option, 0, &Ctrl->D.mode);
	GMT_Report (API, GMT_MSG_ERROR, "Comment -D: Not implemented yet.\n");
	break;
	case 'E': /* Evaluate on ellipsoid */
	n_errors += gmt_M_repeated_module_option (API, Ctrl->E.active);
	GMT_Report (API, GMT_MSG_ERROR, "Comment -E: Not implemented yet.\n");
	break;
	case 'F': /* Bandpass or Gaussian filter -F[k]<lc>/<lp>/<hp>/<hc> or -F[k]<lo>/<hi>*/

Description of proposed changes

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.

[seisman]

Perhaps we should remove the options if they are not implemented.

[joa-quim]

Yes, that would be better. From memory sph2grd is a very broken module.