Add default reference point for map inset (-D)

[Esteban82]

Assisted-by: Claude Sonnet 4.6

This PR makes the argument optional for the map inset option (-D).

When omitted, GMT now defaults to jTL (top-left corner inside the map frame) instead of raising a parsing error.

Test with:

gmt begin ex44 png
	gmt coast -R110E/170E/44S/9S -JM15c -Gbrown -Baf
    gmt inset begin -D+w4c -F+gwhite+p1p+c0.1c+s -Vi
		gmt coast -Rg -JG120/30S/? -Gbrown -Bg
	gmt inset end
gmt end

Please let me know if you’d prefer a different default setting. In PyGMT the Bottom Left corner is used.

[joa-quim]

Maybe it's only my taste, but I would put it at TR (that's what you did in the other PR) or BR

[Esteban82]

Yes, I would put it there too, but it would be the same corner used by the directional arrow (in #9074). That's why I chose "top left" so that the default positions wouldn't overlap.

[joa-quim]

Well, in that (very very rare) case the user has to decide where he/she wants to pout each of them.

[Esteban82]

I don't think it's that rare to have both an inset and a north arrow on the same map.

But if you insist with TR, I can change it.

[Esteban82]

Changed to TR.

[yvonnefroehlich]

For PyGMT we have currently the Bottom Left corner of the plot as default

[Esteban82]

I have no problem to change it. @joa-quim ?

[joa-quim]

GMT.jl has insets too, but more elaborated as they can search best locations, but for full filled base image the default is TR. And the example in the GMT inset page, also set TR as the location. I don't think BL is a common default location for an inset.