Add additional projections

[FirefoxMetzger]

In addition to a perspective projection via perspective_frustum there are a few more functions that are viable to include:

• perspective_focal(focal_length, image_shape) which is the same as perspective_frustum but takes the focal_length directly
• parallel(direction, image_shape) which gives us access to the family of parallel projections
• stereographic(radius, image_shape) which gives us access to stereographic projections
• perspective_frustum in N dimensions

Bonus points if we can write them in a general, N-dimensional way. This would give us some really nice visualization power for higher dimensional spaces (e.g. joint-space). In this case, the proposed signature may have to change.