grdcontour -C & externals is wrong

```
julia> C = makecpt(T=(-7,9,2), Vd=1);
makecpt -T-7/9/2
```
Reading grid from file it's (nearly) OK. Nearly because the aliasing is clearly visible

Now do the same but using data in memory.
```
julia> Gp = gmtread("peaks.grd");

julia> grdcontour(Gp, C=C, N=true, savefig="p_bad.png", show=1, Vd=1)
grdcontour -JX12c/0 -Baf -BWSen -C -N -P -K > C:\TMP\GMTjl_tmp.ps
```

Finally, another bug is when trying to do the same as a modern one liner
```
gmt grdcontour peaks.grd -JX12c/0 -Baf -BWSen -Cc2.cpt -N -png C2
```
Here it creates two separate figures. One with the colors and another with the contours.

The grid is here but not zipped. Just remove the ``.zip`` (needed to cheat github)
[peaks.grd.zip](https://github.com/GenericMappingTools/gmt/files/4957187/peaks.grd.zip)