Task/refactor price import

Bug fix: no
Feature addition: yes
Backwards compatibility break: no
Unit test passes: yes
Behat scenarios passes: yes
Checkstyle issues: no
ChangeLog updated: yes
Documentation PR: 
Fixes the following jira:

- Product fixtures can now contain a single price per column, with the currency as suffix
- YAML product fixtures can now contain an array for prices, with the currency as key