Update 10.2.2013 1146

datagrid_fast_2d.h:		Modified interpolation algorithm to correctly find data points when close to
						boundaries and reverted field calculations back to previous version.
datagrid_test.cc:		Created a simple test to verify accuracy of both data_grid and fast_2d. Test
						uses a third-degree polynomial as a generating function for the field. The
						accuracy of each method is then tested against this generating function.