Fix RawMatrix API

* RawMatrix.Matrix was a mistake and is removed.
* Load has an implication that the underlying RawMatrix would not change, this was not true, so change the name and update the docs.
Maybe RawMatrixChanger?

A RawMatrixChanger can change the underlying RawMatrix used by the reciever. (then what you say, except reflected in RawMatrix.Data (no the)).