Add a method to the `CRS` class called `getElement` that takes
a row index `i` and a column index `j` and returns
the element in the sparse matrix that is in row `i` and
column `j`.
The first line of this method would be

float getElement(int i, int j)

**Hint:** You should start by examining the `row_ptr` array
to find where the `i`th row starts and where it ends in the array
`val`. Then search the appropriate portion of the
`col_idx` array to find an element with column index `j`.