Added optional argument to rake import task to select the base year of a dataset.

As explained in https://github.com/quintel/etsource/issues/812, we now have multiple datasets for the same country, based on data from different base years. I added an optional argument BASE_YEAR to the `rake import` task that can be used to select the year for which to import the data. If you don't specify BASE_YEAR, it will default to 2012. Be aware that not all datasets have a 2012 version yet! 

Closes https://github.com/quintel/etsource/issues/812.