Fix behavior of `NormSqFunctional` for non-zero imaginary parts.

The definition used before is inconsistent for complex numbers, e.g. nsq = NormSqFunctional; nsq.evaluate(i), nsq.evaluate(1) returns -0.5 and 0.5. Remark that A' is conjugate-transpose and A.' is transpose of A.

The behavior for real inputs is unaltered.

Tikhonov reconstructions aren't affected because only zero-imaginary parts occur.