PIM-3541 : introduce an interface for Product Group entity
PIM-3541 : use the new GroupInterface
PIM-3541 : introduce a group factory to avoid to use a hardcoded new
Group
Merge pull request #1899 from akeneo/PIM-3541

PIM-3541 : introduce and use a GroupInterface
PIM-2451 : first POC of product template storage
PIM-2451 : declare denormalizers, hot fix the product value and media
PIM-2451 : update the demo command
PIM-2451 : add a detacher to detach an object
PIM-2451 : fix the error msg