Release version 0.4.0

New features
------------

- The narrowband-to-broadband conversion model for the MSI VNS channels is now
able to use multiple sets of coefficients, one set per solar zenith angle bin.
There are two changes in the config file to reflect this:
`coefficients_nb2bb_vns` now contains a set of regression coefficients per
solar zenith angle bin, so the number of dimensions is now 3, and a new
variable `coefficients_nb2bb_vns_solar_zenith_angles` has been added,
specifying the end points of the solar zenith angle bins. Default coefficients
from theoretical regressions for the range 0° to 80° have been added. Note that
the previous coefficients corresponded to the current coefficients at a solar
zenith angle of 30°.
- The masking behaviour of the VNS narrowband-to-broadband conversion functions
has changed. Broadband quantities derived from the VNS channels will now always
be masked outside of the range of solar zenith angles specified in the
narrowband-to-broadband model. For the default config file, this means that
values will be masked for solar zenith angles between 80° and 90°.

Changes breaking backward compatibility of the interface
--------------------------------------------------------

None.

Other changes
-------------

- Fixed a bug in the conversion from brightness temperatures to radiances: the
code would silently extrapolate beyond the upper end of the look-up table. Now
it masks the data instead.
- Avoid a rare error condition where an exception would be raised in the
destructor of EcaioBase. It didn't seem to do any harm, but it shouldn't raise
an exception, either.
- The data model has been reviewed. While this doesn't affect users directly,
it does open the door for more powerful testing.
- When a variable can't be found in an EarthCARE product, the exception that is
raised is now of a custom type, whereas before it used to be of type
`AttributeError`. I never advertised the type of this exception, so I don't
consider this to be a breaking change for users.
- Fixed a bug where the narrowband-to-broadband unfiltered solar radiance
wasn't exactly zero at night.