fix memory leaks in nonlinear operator chains
fix memory leaks in nonlinear operator chains

sdimoudi/bart,mrirecon/bart,sidward/bart,Takishima/bart,sidward/bart,sidward/bart,sdimoudi/bart,Takishima/bart,sdimoudi/bart,mrirecon/bart,sidward/bart,Takishima/bart,sdimoudi/bart,Takishima/bart,sdimoudi/bart,Takishima/bart,mrirecon/bart,sidward/bart,mrirecon/bart,mrirecon/bart