WIP Testing meca -I to place symbols in a legend or box

[PaulWessel]

Description of proposed changes

See #4445. This PR tests this concept. For example you can try this command and change the -I choices:

echo -159.70 54.81 39 0.82 -0.68 -0.14 0.63 0.34 -0.40 25 | gmt meca -R0/10/0/5 -Jx1c -B0 -Sm2.5c -M -G110/168/255 -T0 -W0.5p -IjTR+o0.25c -png map

Few things to know:

1. Regardless of input file, only the first record is plotted - we then skip the rest
2. The symbol size is what -S gives - no scaling by magnitude
3. The -N is turned on since -IJ will place things outside rather than inside the box
4. While -Ix should not need -R -J I have not implemented that yet since affects THIS_MODULE_NEEDS
5. The -C option should be disallowed or ignored when -I is used.

We should make a test page of all the 9 meca symbols using the 9 adjustment points.

[PaulWessel]

Hi @seisman, I think you voted this down when I put this together. If that is still the sentiment then I can delete this branch, yes?

[seisman]

If I understand it correctly, after implementing the new legend code proposed in #4445 (comment), the -I option implemented in this PR is no longer needed. If anyone wants to place a beachball at the top-right corner, he can always pass the new legend code to legend.

[PaulWessel]

Ah yes, I hate these abandoned issues of mine... Thanks for reminding me. Yes this is the better solution but not implemented yet. I think I will wait with deleting the branch in case I may reuse some of the coding there for this purpose.