Bounds checking for `_entry` function

I was trying to implement the `_entry` function for `ElemWise` class. I realized all the `_entry` functions don't check for bounds and simply return `MatrixElement(self, i, j)`
```
In [5]: A = MatrixSymbol('A', 3, 3)

In [6]: A._entry(5, 6)
Out[6]: A[5, 6]
```
I understand this is an internal function but I felt the need for this.