Add Stan models for unit vector transforms

[sethaxen]

Adds Stan models for 2 transforms to unit vectors:

• embedded space with normal prior
• hyperspherical coordinates (with interval transforms for most angles and embedded space for remaining parameter)

The paper includes the derivations.

There are several definitions of hyperspherical coordinates. I picked a particular one, but maybe someone prefers a different convention?

[sethaxen]

Relates #22

[spinkney]

Neat way of getting the 2 final parameters into $[0, 2\pi]$