Add option to dataset to pass separate arrays of x, y, and z

[joshua-gould]

We could convert Dataset to an interface with getX(index), getY(index), getZ(index) methods so users could store positions how they desire.

[cannoneyed]

Are you running into issues with this, eg is your data in separated format? I'll have to think through the consequences of this one a bit...

[joshua-gould]

My data is in separate arrays. It is easy to convert to the format that Dataset expects, but makes initialization slightly slower.