Make inclination theta jn

There's been a longstanding issue that we call us iota instead of theta_jn. We should really fix this for consistency with other codes.

This does that. It will not be backward compatible. You cannot now sample in iota you must sample in theta_jn. This shouldn't mess up too many people unless they've made their own prior files.

We may want to update the documentation to have a detailed description of what all of the parameters mean.