Filter on years when converting model files

[willu47]

It would be useful to reduce the size of the model file e.g. generate for just one or three years, by adding an argument e.g. --min_year, --max_year

otoole convert datapackage datafile simplicity/datapackage.json simplicity.txt --min_year 2015 --max_year 2016

Implementation

These arguments could be passed to the functions which create the Pandas Datatables, filtering on the arguments. First, the values should be checked against the YEAR set.