Shift and add

[matteobachetti]

Basic implementation of the shift-and-add technique from Mendez+1998 Guide for review: the basic mechanism is in fourier.py, the examples in the doctests should be self-explanatory. InDynamicalCrossspectrum and DynamicalPowerspectrum, I just wrap the same functionality in convenient methods that unpack the relevant information from the dynamical spectra and use the function in fourier.py.

Example: if a QPO is changing its frequency during our observation

we can think of tracing its maximum (e.g. with dcs.trace_maximum(), here I did it by hand):

and then applying shift_and_add to improve signal-to-noise and, maybe, detect a second QPO:

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.11%. Comparing base (39431fc) to head (7b7585d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #849 +/- ## ========================================== + Coverage 96.08% 96.11% +0.02% ========================================== Files 48 48 Lines 9292 9359 +67 ========================================== + Hits 8928 8995 +67 Misses 364 364 ```

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