Add SICP exercise 3.26
Add SICP exercise 3.27
Add SICP exercise 3.28
Add SICP exercise 3.29
Add alternative or-gate graph and delay result in ex3.29
Add SICP exercise 3.30
Add SICP exercise 3.31 test suite
Add propagate, inverter, and-gate, or-gate test case on ex3_31 test file
Add SICP exercise 3.31