Adjustable following distance w/ new MPC

[sshane]

Experimental and may crash, don't use this branch

Crash test results with various distances:

WARNING: deceleration tests with these costs:
0.9 TR gives FCW at 2 m/s/s test and fails at 3 m/s/s test
1.2 TR fails at 3 m/s/s test
1.4 TR gives FCW at 3 m/s/s test and fails at 3+ m/s/s test
1.6 TR succeeds at 3+ m/s/s test without FCW