add unit/integrted tests for dpc

I think the previous code block is problematic:
In (4), div_v might have 0-value elements;
(5) is redundant and can be removed.
So I adopted your codes and there is no need to bench mark. Do you agree?
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(1) c = -1j * (kappax * tx + w * kappay * ty)
(2) div_v = kappax**2 + w * kappay**2
(3) zero_arr = (div_v == 0)
(4) c /= div_v
(5) c[zero_arr] = 0