PIM-1401 Fixed displayed conversion unit fields for a given channel

In fact, one field should be displayed for each metric attributes (and
not one field per metric family). This allows to define two different
conversion units for two metric attributes that are using the same
metric family.
Merge branch 'master' into feature/pim-1401
Added metric values in demo fixtures
PIM-1401 Fixed typo
Fixed convert metric feature description
Merge pull request #548 from akeneo/feature/pim-1401

PIM-1401 Fixed displayed conversion unit fields for a given channel