Add aliases for latest version of available datasets

[jucordero]

In cases where multiple releases exists (e.g. FAOSTAT, UN population), importing the named dataset should return the latest version of the data, with older version available by importing via an alias with the year appended or some other identifier to tell versions apart.

e.g.

from agrifoodpy_data.food import FAOSTAT

should import the latest version of the FAOSTAT dataset, while

from agrifoodpy_data.food import FAOSTAT_2020

should return the 2020 version of the dataset.

It is likely that the difference between these two will be just the new data being appended.
But in some cases data can change retroactively, which would be nice to keep track of.

[jucordero]

This can wait for now, as we only have a single version of each dataset processed.
FAOSTAT, UN population, and a few geospatial datasets have been updated since so we could start by ingesting these first.