SCUBA2: add units to log file, document additional entries

The units can now be given to the log file entry in order to ensure
values are of the expected magnitude.

Also I'd missed documenting the Peak_obs and Peak_fit parameters.
SCUBA2: add a units line to the header

The header of the log file now contains the units of the default
entries, to which users can add their own for their particular log
file.