clean up and refactor. Old interface un-changed.
- manntensor.py refactored to avoid duplicate code.
- refactor SpectralTensor to
- MannTurbulenceInput with methods spectra_integrated, spectra_lookup, spectrum_variance and spectrum_TI
- SpectralTensor (inherrits MannTurbulenceInput) with methods generate_spectral_tensor (tensor is stored for faster generation of boxes with different seeds and alphaepsilon) and generate_field)
- OnetimeSpectralTensor (inherrits SpectralTensor, but does not store tensor)