Added GET and SET functions

Operrator() has been overloaded to support
operations like 'cout << mat(1,2)' and
'mat(1,2) = 2'.
Code for GET and SET operations

Operator () for rvalue and lvalue operations
has been overloaded. <stdexcept> header file
is now included so that out_of_range exception
is thrown if index are out of bounds.
Output for SET and GET operations

main was changed to test the changes corres-
ponding to the overloaded () operators.