Let the pulse coeff handle complex coefficients

[Wikstahl]

This fixes issue #148

[BoxiLi]

Hi @Wikstahl, thanks a lot for the PR!

It is probably better to set the dtype of the new coefficient array to be the same as the old one, instead of always using a complex array.

Could you also add a new test for the complex coefficients?

[Wikstahl]

Sure I can fix this! :)

[BoxiLi]

I just pushed a patch with dtype=old_coeffs.dtype

This should make the test pass. However, it does not mean that the complex coefficient is fully supported. The Processor.plot_pulse will still get a warning because of the complex numbers. But this at least lets the computation run without an error.

In fact, one can always rewrite complex coefficients by real ones with two sx and sy control terms. So real coefficient also has no loss of generality.