Adding support for MERIS IODD8

At least reading the SNOW_ICE MERIS L2 flag should work now.
This should be checked by experts though.
(I added TODOs in various places.)

The main change was:
- to rename MER_RR__2P_flags_data to MER_RR__2P_flags_IODD7_data
- to derive a new version MER_RR__2P_flags_data from it:
   - Added SNOW_ICE
   - Replaced LARS_ON by DDV
   - Removed LOW_PRESSURE
   - Fixed typo in description of WHITE_SCATTERER (Presense -> Presence)
   - Fixed bug (?) in CASE2_S: Using bits 8+21 (was: 8 only, i.e. water bit 21 not set)

The following references were used for this change:
- http://www.brockmann-consult.de/beam-jira/browse/BEAM-1237?page=com.atlassian.jira.plugin.system.issuetabpanels:changehistory-tabpanel
- https://github.com/bcdev/beam/blob/58f66c3d6697daab049abaa7284b26f652bfca5c/beam-envisat-reader/src/main/resources/org/esa/beam/resources/dddb/flags/MER_RR__2P_flags.dd
- http://earth.eo.esa.int/pcs/envisat/meris/documentation/MERIS_IPF_evolution.pdf