The convertCurrency hook will be triggered if your store configuration
uses custom currency conversion provider (i.e. other than ecb.int or admin.ch that Isotope ships by default).
This hook will be triggered by Isotope Automator according to cronjob schedule (daily by default),
and this is where you should pull the up-to-date exchange rate and save it to the database.
Parameter
\Isotope\Model\Config $config
The instance of the Config model. The hook is called once for each store config that
has custom provider set up.