Add SpectralTensor.spectra, remove seed loop in SpectralTensor.generate_field and add several tests
- Add SpectralTensor.spectra -> theoretical spectra
- Add turb_utils.spectra -> spectra of turbulence box
- Remove seed loop in SpectralTensor.generate_field as it is already in outer generate_field-function
- Tests:
- Increase size of box in test_against_reference
- test_against_reference with and without hfc
- test integrated spectra vs look-up table
- test spectra for dx=[1,2,4]
- test coherence (extracted from Test_Turbgen_Coherence.py)