This commit is trying to fix these bugs:

PercolationStat.java: 
  - Test 8: Check that the methods return the same value, regardless of the order in which they are called
  

Percolation.java:
  - Test 1: Check whether exception is called if (i, j) are out of bounds
  - Test 2: Open predetermined list of sites using files
  - Test 4: Opens predetermined sites, but where N = 1 and N = 2 (corner case test)
  - Test 6: Check for backwash with predetermined sites that havemultiple percolating paths