Ability to place seismological/geological symbols in legends

**Description of the desired feature**

Modules like **meca**, **velo**, etc. plot advanced symbols in seismology and geodesy and use a geographic region (**-R**), map projection (**-J)** and input _longitude, latitude_ to place these features on a map. There are times when we also would like to plot such a symbol outside a map, such as in a legend. In legends, there usually is no geographic region nor projection, and at best there is a Cartesian **-R -J** set up for placing information. Because the above-mentioned tools reads longitude, latitude of where a feature is it becomes a guessing game to pick **-R -J** and input coordinates to align the symbol, say, centered in a legend.

One way to handle this would be to add a new option to these modules that, like **-D** for colorbars or scales, deals with this issue. Examining the various modules I see that the option **-I** (for information) is available [or **-Z** if **coupe** is can be excluded]. Thus, a solution may be this:

1. Add **-I**[**g**|**j**|**J**|**n**|**x**]_refpoint_ for specifying where the special symbol should be plotted (with the usual ways of _refpoint_ values like coordinates or codes)
2. When **-I** is used the input longitude, latitude is ignored and placement is given via **-I** instead.

For instance, to center a focal mechanism in the middle of a 5x10 cm legend area we could do

```
gmt meca -R0/10/0/5 -Jx1c -Sm2.5c -M -G110/168/255 -T0 -W0.5p -Yh+0.5c -IjCM <<- EOF
-159.70 54.81 39 0.82 -0.68 -0.14 0.63 0.34 -0.40 25
EOF
```

and the actual earthquake location (`-159.70 54.81 `) is simply ignored. I think the proposed mechanism will make it easier to add special symbols such as GPS velocity with error symbols and focal mechanism to user legends.

Comments, @joa-quim and @seisman? Apart from meca and velo, does it make sense to extend this solution to **polar**, **coupe**, and **sac**?