Additional angular power spectra

Detset Angular Power spectra

While the HFI is not providing sky maps obtained with different subsets of the detectors in a given frequency channel (see Detset maps), it provides auto-spectra and cross-spectra derived from these maps.

Outstanding:

• KB and/or EFH to add description of auto & cross spectra and of the associated covariance matrices
• AMo to add description of file with sample header