Fix what is probably a typo.
'definition' parameter in constructor doesn't have a 'name' key.

Use the 'name' parameter instead.

Add tests to cover the changes.
Add a validation routine for group definitions.

Assume an empty group and a group declaring unknown fixtures are
forbidden.
Merge pull request #2 from GeJ/PR_Definition_patch

Thanks for the patches!

Definition : fix a few typos and add a couple of restrictions on groups