If future coverage of the city's often-precarious finances gets even more difficult to understand, there's a reason for it: Uri Monson, executive director of the state board overseeing city finances, the Pennsylvania Intergovernmental Cooperation Authority, is giving up the job. He's held PICA's top staff position for four years, succeeding Rob Dubow when Dubow became city finance director. For years before that, Monson has been a patient, reliable font of information on revenue forecasts, pension performance, collective-bargaining requirements and the other arcane budget issues.    No word yet on Monson's future plans or how PICA will replace him.