create C1 = 2.0*C1 + C, C1 has less non-zeros than C 
 MatAXPY(C1,2.0,C,DIFFERENT_NONZERO_PATTERN)...
 C1: nz_allocated = 20.; nz_used = 20.; nz_unneeded = 0.

create C2 = 2.0*C2 + C, C2 has non-zero pattern of C2 + C 
 MatAXPY(C2,2.0,C,SUBSET_NONZERO_PATTERN)...
 C2: nz_allocated = 20.; nz_used = 20.; nz_unneeded = 0.