New issue based on in-flight experience (Performance Verification phase and Science Demonstration phase): all sections re-written.

Various minor corrections and typo/formatting corrections.

Analytic formula to compute sensitivity in mini-scan map mode given in Section 5.2

Warning added in Section 5.1.3 that the column "central area point-source sensitivity" is only applicable in the mini-scan map mode.

Various minor updates on the use of the scan-mapping and its performances.

Section 4.4 updated point-source correction factors

Section 4.6 observed beam maps and beam efficiency figures added

Section 4.9 on second-pass spectral ghosts introduced

Section 4.10 Spectrometer flux calibration accuracies updated

Section 4.11 unchopped sensitivity plots added

Section 4.13 new section on pointing accuracy and impact on data

Section 6.1 updates in unchopped line spectroscopy AOT usage

Section 6.2 updates in unchopped range spectroscopy AOT usage