if the user sets an implementation for SpMV using the  compute_with field, this is now checked whether it is a valid implementation
if the user sets an implementation for SpMV using the  compute_with field, this is now checked whether it is a valid implementation

RRZE-HPC/GHOST,RRZE-HPC/GHOST,RRZE-HPC/GHOST,RRZE-HPC/GHOST