Create a notebook to showcase usage of Cryosat and CHAMP in particular

[smithara]

Now that CHAMP (2000-2010) and Cryosat (2010-onward) are available, these need to be documented better. It's an interesting opportunity to demonstrate two solar cycles of data.

Include background and usage notes from Nils about Cryosat:

There are many processing changes compared to the previous data releases, including
a) use of improved positions (instead of the onboard position solution)
b) magnetometer data shifted in time by 0.6 sec (a scalar calibration including time shift resulted in minimum rms misfit with this value, consistently when looking at various months worth of data)
c) improved attitude processing (adopted from Swarm L1b processing) including smoothing in time.

In addition, we used a new data pipeline, which we also plan to use for providing new Cryosat data on a more frequently basis, striving at monthly updates.

The quality of the attitude - and thus of the vector data in the NEC frame - is given by the quantity "q_error"; we recommend to only use vector data when q_error < 20.
Note that magnetic field intensity is still valid even if q_error > 20.

The "cumulative distribution function" (cdf) of the daily rms misfit (wrt CHAOS 7.18, for QD-latitudes equatorward of 60 degrees) shown below indicates good quality of the data (median rms values of 7.4 nT for the horizontal components 5.0 - 5.2 nT for the vertical component and F, respectively). Among the three magnetometers, FGM1 is likely the best one while FGM2 is worst.

Some of this extra information needs to be preserved in the handbook.