Task/refactor import

Bug fix: no
Feature addition: yes
Backwards compatibility break: yes
Unit test passes: yes
Behat scenarios passes: yes
Checkstyle issues: no
ChangeLog updated: yes
Documentation PR: 
Fixes the following jira:

This PR refactors imports by removing a couple unneeded abstract classes. It also adds the following features :

- a new processor and fixture file format for product associations (could be easily used to make an independent product association import)
- product CSV import files are now read only once. Step2 of product import is only run if association columns are present
- added missing unique index for product associations
- suffixe for product associations import are now "-groups" and "-products"