Add non-geographic plots to projection gallery

[willschlitzer]

I added cartesian.py to have a Cartesian example for the projection gallery for #292.

[willschlitzer]

I'm looking to add a polar coordinate plot example to the non-geographic gallery, but can't figure out how to plot it. I can use the P argument but am not sure how to add the other inputs, as the example code on the GMT docs uses grdmath, which doesn't look like an option on PyGMT at this time. I'll add polar plotting to the pull request if I can figure it out.

[seisman]

how to add the other inputs

You can simply pass x and y (i.e., angle theta and radius r) to the plot function.