Document water transport, including pref flow

In order to use PREF_FLOW, a new column must be present in the sno files. This should be documented somewhere (as well as a note in Inishell) and checked in the SNO plugin. Most probably, the best option would be:

• write a documentation in WaterTransport.cc that explains how water transport works and how ti can be configured (with links to the papers)
• implement checks in SmetIO.cc and other plugins (if any) that read SNO data to make sure that when such an option is toggled, the user will get an appropriate error message if the SNO files does not contain the necessary columns
• add some information about the SNO files in Inishell

The same should also be done with SEA_ICE...