Improvements on Signal Initialization in Testbench Generation

[tangxifan]

To address #128

• Now signal initialization is only applied to datapath inputs of MUX2 cells and pass-gate logics.
• Now use $urandom to generate 1-bit random value when required for signal initialization.
• Update documentation to clarify the port sequence requirements by signal initialization for the circuit models about MUX2 cells and and pass-gate logics.