Add methods to shift plot origins

[seisman]

Description of the desired feature

In GMT command line, -X and -Y are used to shift plot origins. Sometimes, we need to shift origins but doesn't plot anything. We can use gmt plot -T -X10c -Y10c to shift the origins without plotting anything. However, it's not intuitive both in gmt cmd and gmt-python.

We can add some new methods to shift the origins more explicitly, e.g.

fig.shift_origin(xshift="10c", yshift="10c")

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

[leouieda]

Sorry, forgot about this and opened #267. Will close this one but keep a reference there.