[ENH] Spectra transformers

[ngergihun]

This pull request introduces several new spectrum transformation classes and extends the test coverage for these transformations in the pySNOM package. The main additions are the Cut, Scale, and ConstantNormalize transformations, as well as enhancements to the RotatePhase transformation. Comprehensive unit tests have been added to ensure correctness and reliability of these new and modified features.

New spectrum transformation classes:

• Added the Cut class to extract a spectral region between two wavenumbers from a spectrum (pySNOM/spectra.py).
• Added the Scale class to scale amplitude or phase data by a specified factor (pySNOM/spectra.py).
• Added the ConstantNormalize class for normalization by a constant value or by a value from the spectrum at a specific wavenumber, supporting both amplitude and phase data (pySNOM/spectra.py).

Enhancements to existing transformations:

• Modified the RotatePhase class to support an optional constant phase shift in addition to the wavenumber-scaled phase shift (pySNOM/spectra.py).

Test coverage improvements:

• Added comprehensive unit tests for the new Cut, Scale, and ConstantNormalize transformations, as well as for the enhanced RotatePhase and other normalization methods (pySNOM/tests/test_spectra.py).