Add default reference point for map rose ( -Td and -Tm)

[Esteban82]

This PR makes the argument optional for the map rose options (-Td and -Tm).

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

This behavior can be tested with:

gmt basemap -R-50/-40/-20/-18 -JM10c -Bf -Td+f -png test

Feedback on the default position is welcome.

[joa-quim]

Top-Right seems good to me.

[Esteban82]

Top-Right seems good to me.

Ok. But I think that in pygmt it is at the bottom left corner by default (link). Is this correct @seisman? Should we use the same default position? Or is it ok to have different positions for GMT and PyGMT?

[joa-quim]

Asked Google and it came out with

The traditional symbol for directions on a map (the compass rose) is typically placed in the upper right corner. Placing it here keeps it out of the way of the main map details while remaining easily visible. If an upper corner is unavailable, it is placed in the lower right or lower left.

I think UR is the best default location.

[Esteban82]

Fine by me.