Speed up fourier py

[matteobachetti]

In generate_indices_of_segment_boundaries_unbinned:

• [x] Avoid unnecessary check for array sortedness. This saves ~1/3 of computing time in large, memory mapped arrays
• [x] Use numpy.searchsorted only once

[codecov[bot]]

Codecov Report

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

Project coverage is 96.43%. Comparing base (7377187) to head (9341db0).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #808 +/- ## ======================================= Coverage 96.42% 96.43% ======================================= Files 45 45 Lines 9016 9027 +11 ======================================= + Hits 8694 8705 +11 Misses 322 322 ```

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