#77 created class WrongMapperError and made nansat handle this-one-only when searching for mappers
#77 adjusted mapper ncep_wind_online to raise WrongMapperError (including test raise of IOError). The other mappers also need to be updated before nansat will work again.
#77 and #78: implemented use of nansat.tools.WrongMapperError in all mappers. Also solved #78 for Envisat products.
forgot to add self.product in Envisat.__init__
#77 Add generic mapper test.