[Incomplete] Document Data Structures

Hi David,

Sorry for the sudden hiatus... work was killing me
I'd like to gradually add some of the code I've written for work into breeze and get into contributing again.
For that, I think I really should understand the inheritance structure of DenseVector and DenseMatrix.

I've started an ambitious attempt to take notes and make a UML-ish diagram... do you think something like the following would be useful to really make complete?
![image](https://cloud.githubusercontent.com/assets/4279323/3775458/419e2098-1945-11e4-9190-1a6eb0bbc064.png)

I'd like to go through the classes and add more docs with your help, as well as make the variable/type anno naming consistent, if that's OK with you?

Sincerely,
Kenta
@dlwh 
I'm going to now try to organize the inheritance hierarchy for DenseMatrix:
(in remote preparation for adding 3+ dimensional stuff)

![image](https://cloud.githubusercontent.com/assets/4279323/3787500/014085d4-1a1e-11e4-9263-96545e0c35a9.png)
(I abused the symbols to trick the "genmymodel" site: the white arrowhead is inherit, the dotted line arrow are traits used with "with", and the black square arrowheads mean type annotation with `{ this: DenseMatrix. type =>`

I know we discussed this a bit before:
https://github.com/scalanlp/breeze/pull/232
I guess there is no real way to test any restructuring I do... 
do you have any pointers for how to streamline this?
do you think I should just keep my hands off?