Add default position for map scales when refpoint is omitted

[Esteban82]

Assisted by Claude Sonnet 4.6

This PR adds a default placement for map scales when refpoint is omitted. The scale is automatically placed in the lower-right corner of the map (+jBR+o0.5c), slightly offset from the frame.

Testing
gmt basemap -R0/10/0/1 -JM10c -Baf -L+f+w150k -png test

Let me know if you prefer another position.

[joa-quim]

Hmm, sorry disagreeing again but this is a case where the default must clearly be LL.

[Esteban82]

Ok. I will change to LL. Thanks for the feedback.

[Esteban82]

I just changed to jBL.

[Esteban82]

Now I got this figure.

[yvonnefroehlich]

In PyGMT we have for the high-level method Figure.scalebar:

If not specified, defaults to the Bottom Left corner of the plot with a 0.2-cm and 0.4-cm offset in the x- and y-directions, respectively.

as default.

[Esteban82]

Okay, I'll update the offset with those values. I suspect they were intended to be higher than the one I used. Thanks Yvonne for letting me know.

[Esteban82]

Offset changed.

gmt basemap -R0/10/0/2 -JM10c -Baf -L+f+w150k -png test