Let -R (-Ra or -R{dcw}) handle Z depth

[gd-a]

Hi there,

simple context

Could you add the possibility to handle 3D coordinates through an option when using -Ra or -R{dcw code}.

Examples given :

gmt plot3d -Ra+z0/200 -JM10c -JZ1c -Sc0.1c -Gred -Bafg -p210/30 -png new_option << EOF
-60.638500 14.052167 6.36
-60.558833 15.332167 30.0
-62.130833 17.402000 46.0
-61.890667 16.472167 150.51
-61.902333 17.504500 0.0
-60.722167 15.989667 29.08
-61.173667 15.327667 140.55
-62.981667 17.400167 169.77
EOF

or

gmt plot3d -RGP,MQ+z -JM10c -JZ1c -Sc0.1c -Gred -Bafg -p210/30 -png new_option2 << EOF
-60.638500 14.052167 6.36
-60.558833 15.332167 30.0
-62.130833 17.402000 46.0
-61.890667 16.472167 150.51
-61.902333 17.504500 0.0
-60.722167 15.989667 29.08
-61.173667 15.327667 140.55
-62.981667 17.400167 169.77
EOF

Thanks :)