The renowned percussion group NEXUS, consisting of Bob Becker, Bill Cahn, Russell Hartenberger and Gary Kvistad made a Southern California appearance Friday night before a noticeably full Samuelson Chapel at California Lutheran University. A bewildering array of xylophones, marimbas, bells, cymbals and drums of every description crowded the stage for the two hour performance. NEXUS has been making contemporary music since 1971 and has been a presence worldwide. The entire first half of the concert was devoted to the music of Steve Reich, starting with his Music for Pieces of Wood written in 1973. This is performed on tuned wooden