Neshaminy School District teachers say they are ready to come off the picket line but there are some strings attached.   They want around the clock negotiations for a week and binding arbitration if those talks fail.    School District officials are viewing the union offer as a formal request for binding arbitration and say the teachers should return to work while the school board considers the proposal.   But the union doesn't see it that way and isn't likely to call off the strike in time for students to be back in the classroom on Tuesday.   Negotiations have dragged on for almost four years.