Add option for computing cross spectral density
At some points, we want to compute the cross spectral density (CSD) between two time-series data file. For example to test the transfer function in CSD of the TDI XY channel. The expression by definition for the CSD is
S_{xy} (f) = \tilde{x}^*(f) \times \tilde{y}(f)
We now study to implement that feature (TBC).