Description of the desired feature

Implement grdview which "Creates 3-D perspective image or surface mesh from a grid". Basically a 3D version of 'grdimage` that was wrapped in #124. Below is an example of such a 3D plot:

The grdview function fits in with the rest of the Plotting modules and shouldn't be too hard to implement. I've created heaps of 3D perspective plots before for my thesis using GMT5 and definitely found the plot quality and features a lot nicer than matplotlib. This will be a great addition for people working with contours and DEM surfaces and brings PyGMT into the 3D realm!

Code-wise, I'll stick to using the earth_relief_grid for the unit tests. As for the aliases, I'm thinking of using 'perspective' for -p and 'zscaling' for -Jz (though unsure how to handle -JZ). -Q seems to be worth aliasing but I can't come up with a good name...

Are you willing to help implement and maintain this feature? Yes