Rename package groupId

[sam]

Right now we're using org.sam.doubleshot. The groupId needs to be a domain-name I control however according to the Maven Central publishing guidelines.

For hosted projects we have the option of using something like com.github.sam, but I'm not in love with the idea of namespacing the code to github. So instead we'll change it to org.substantiality, which is a personal domain I've had for many a year.