Added vector class documentation.

I'm not completely sure on my understanding of `MockVector`. Also my explanation of a `SparseVector` is kinda ehh...

Refer to #208 
Just FYI, if these were meant to be separate paragraphs when the javadoc is rendered, simply doing an extra line here does not do what you want. You need to add a paragraph break `<p/>`:
```
elements take up memory, space, try a sparse structure.
<p/>
However, fetch/store...
```