Adding command &filter_equiv to filter candidate equivalence classes using indexes of disproved POs after handling SRM as a multi-output miter.

Added new Python API is_const_po( int iPoNum ), which returns 0/1 if current network is an AIG and the given PO has const 0/1 function.

1. Replace system() with a function that responds to SIGINT. 2. Add functions to cleanup temporary files on SIGINT. 3. Fix bugs related to signal handling.

pyabc: make the SIGINT signal handler clean up by sending SIGINT to child processes registered by the python code. Also provide functions to block SIGINT and unblock it, to allow for critical sections where signals are blocked