Read in pixel array by following these instructions:
 *
 * 1. First, allocate space for m `struct pixel *` values, where m is the 
 *    height of the image.  Each pointer will eventually point to one row of 
 *    pixel data.
 * 2. For each pointer you just allocated, initialize it to point to 
 *    heap-allocated space for an entire row of pixel data.
 * 3. Use the given file and pixel_array_offset to initialize the actual 
 *    struct pixel values. Assume that `sizeof(struct pixel) == 3`, which is 
 *    consistent with the bitmap file format.
 *    NOTE: We've tested this assumption on the Teaching Lab machines, but 
 *    if you're trying to work on your own computer, we strongly recommend 
 *    checking this assumption!
 * 4. Return the address of the first `struct pixel *` you initialized.