the main processing steps in rdr2sdr are now:

  - load the matlab RDR data
  - process sci and eng packets
  - order and validate igm data
  - group igm data into scans
  - take igms to count spectra
  - calculate moving averages
  - L1a to L1b calibration

updated scanorder and igm2spec to work in this context and
fixed an oversight in scantail to trim tailing partial scans