Merce Cunningham (1919-2009) April 16 marked the centennial of choreographer Merce Cunningham's birth.  His artistic legacy is that his modern dances, many set to music by his creative and life partner John Cage, will forever be modern. Following Cunningham's directive, his company disbanded after his death at 90 - but not the teaching, training, archiving and performing spanning 70+ years.  The Merce Cunningham Trust's world-wide celebration included a definitive program April 17-21 at the Joyce of three classics performed by three companies.   The Merce Cunningham Trust defines the Cunningham Technique as "a rigorous form of training designed to create strength and flexibility of both the body and