Log Message: 
* Added a new primitive: ACOS. It's the arc cosine function.
* Fixed a bug where in generating points in a disc, if the vector to the center of the disc was parallel to the normal, the points would only be generated at the center.
* Added implementations for generating vectors within the remaining domains. Generating a vector within a plane results in an error due to the domain being unbounded.
* Removed the "blob" domain as it has no use outside of generating points, but this effect can be simulated by providing different random distributions to the generation routine. (Not implemented yet)
* Updated the README to reflect changes.
