MERIS IODD8 support (SNOW_ICE flag)

The SNOW_ICE L2 flag introduced in MERIS IODD8 is currently unknown to the epr-api.
IODD7 is the latest supported, and apparently there is some backward compatibility built in for IODD6 and IODD5.

This fork proposes changes that introduce support for IODD8. In particular it is now possible to read the SNOW_ICE flag from a MERIS L2 product file ("MER_RR__2xyz.N1"). More details are in the commit message of the main commit:
https://github.com/Catalysts/epr-api/commit/9c7088cc3b54ebd84ed51b57a1d5ba14dfed9c6f

Note that there are several comments having TODOs, mostly to indicate that the changes should be reviewed by experts and complemented if necessary.

Cheers