Updates and corrections to the HALO cloudmask book chapter

[tmieslinger]

The plots from the cloud mask chapter shall be part of the EUREC4A HALO ESSD paper (in preparation). From an internal review among the co-authors a few suggestions were made that lead to an update here. The main points were:

• we added a VELOX brightness temperature image to the case study plot on Feb 5.
• for the same figure, the VELOX cloud flag was based on the central 10x10 pixels and was extended to 11x11 pixels (~0.57°) to better match the specMACS (central 0.6°) and radar footprints.
• the time series plot had a bug in the errorbar calculation that was fixed
• campaign mean averages shall cover only local research flights
• further clean up and improvements

[tmieslinger]

Thanks @JuleRadtke! I addressed all your comments and especially changed consistently "cloud fraction" to "cloud cover". Tha was a good point.