otultra2: expose the meal and comments as separate field in the CSV.

Idea by Jim Sifferle, implemented in a slightly different way as I
noticed I forgot to quote it properly.
Generalise the meal/comment handling to the Reading object.

While not all readers implement before/after meal notes, it's possible
to implement an heuristics for that.
glucometer.py: if no unit is provided on command line, use the default of the meter.
Change all the internal representations to mg/dL for compatibility with LifeScan.

While mmol/L is the international standard unit, at least LifeScan uses mg/dL in
all their devices, and since they are the ones for which we have protocols for,
we might as well use the same.
Allow sorting the dumped data; sort by date by default.

This changes the dump output for otultra2 devices as they would dump data in
record format, last reading first.
Clean up the code to remove methods that should be functions and imports.