Added Oversampling Feature

[stroblme]

This PR introduces oversampling for the frequency and time domain.
It also allows applying fftshift to get a properly ordered spectrogram.
With this implementation we move away from the pennylane reference (as the api is too limited).
However, we still have a test in place, to ensure that nothing is faulty with our implementation.
As the method now returns coefficients and frequencies, it was renamed to get_spectrum.
With the new oversampling feature, it can happen, that the spectrum has an even length, which would result in a slightly shifted spectrum afterwards. To mitigate this a trim argument was introduced that removes the Nyquist frequency.
This PR also introduces a get_psd method to calculate the power spectral density of a set of given coefficients.

• add test to check against pennylane implementation
• add test to verify oversampling (time domain)
• add test to verify oversampling (frequency domain)
• add test to check that frequencies match coefficients length
• add documentation and notebook

Breaking Changes

• renamed sample_coefficients to get_spectrum

[stroblme]

d537b8d tests failed because of some import errors with pennylane lightning

[majafranz]

Really good work! LGTM.