Add relative plate motions to summary tab

[baagaard-usgs]

Either implement or link to Paul Earle's plate motion calculator. Given the names of plates and a location, it computes the relative plate motion (and direction) using Morvel 2010. It would be helpful to look at the code (Perl?) for this to see how complicated this is.

[paulearle]

Gavin uses a more up to date model. I will email him to see how he gets in the information.