grdmath for externals seems to only accept input grids, not lines

```
julia> G = gmt("grdmath -R60/95/6/24 -I0.1 ? LDIST", mat2ds([73 24; 89 6]))
ERROR: grd_init: input (Array{GMT.GMTdataset,1}) is not a GRID container type
```
bu it works if I first save the the line into a file and next do

```
julia> G = gmt("grdmath -R60/95/6/24 -I0.1 line.dat LDIST")
```