Consider unifying the treatment of complex vectors by either working with real and imaginary parts or with complex values at least internally

[BjoernLudwig]

For instance in the module model_estimation.fit_filter we are converting back and forth those two formats and could probably improve performance by sticking to one form. This requires thorough checks of the formulas for the arithmetic operation though. Other modules are affected as well probably, e.g. the GUM2DFT related stuff.