optional `pyfftw` import for `simulation.fractal.py`

[avalentino]

Description of proposed changes

Implement a fallback mechanism to use scipy.fft in src/mintpy/simulation/fractal.py in case pyfftw is not available.

N.B. I have not removed pyfftw form the list of optional (test) dependencies.

Reminders

• [X] Pass Pre-commit check (green)
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• [x] Pass Circle CI test (green)
• [X] Make sure that your code follows our style. Use the other functions/files as a basis.
• [x] If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.