Add timezone information to timestamps

[gnn]

The timestamp values stored in the database currently don't have timezone information stored with them. This should be changed so that the timestamp fields are created with timezone information in mind, when setting up the database. In order for this to work, I have to figure out whether the time information contained in the NetCDF files contains explicit timezone information or whether an implicit timezone is assumed. This timezone information can then be stored along the with timestamp values when importing the data.