allow aligned spin with a<0

This MR allows us to specify aligned-spin priors. In this case "a", the dimensionless spin, can be negative as long as the tilts are zero.

@grant.meadors if you have other things to contribute on this branch, that'd be great.

I'm happy to merge this if its ready.

Thanks, let's merge but keep the branch.

mentioned in issue #80 (closed)

mentioned in commit 0bf73d27

merged