[ WIP ] Element wise matrix multiplication

An Implementation for element wise matrix multiplication for `MatrixSymbol` 
`._entry` should define the i,j th element of this matrix.  It is what is called when you use `__getitem__`

I expect

```Python
>>> X = MatrixSymbol('X', n ,n)
>>> Y = ElemWise(X, sin)
>>> Y[i, j]
sin(x_ij)
```

So all you need to do is call the function on the entry of the parent matrix.