In the new Philadelphia Weekly: Instant Composers Pool Orchestra Fri.-Sun., Apr. 1-3, 8pm. $12 ($30 three-night pass). Christ Church Neighborhood House, 20 N. American St. 215.922.1695 www.arsnovaworkshop.org Founded in 1967, this Amsterdam-based tentet still boasts the keen, pranksterish talents of pianist Misha Mengelberg and drummer Han Bennink, the godfathers of Dutch progressive jazz. The group last came to Philly in