Refactor schema/create-dimension!

Moves groups validation to api front
Requires group_only or grouped_by option to be passed
Refactor: extract execute-with-transaction
Add bang to execute-with-transaction
Remove :pre from add-dimension!

The default should be (when neither of both options were passed):

        `{:group_only false :grouped_by [[]]`

The precondition was preventing assuming that default
Merge pull request #7 from nberger/refactor-create-dimension

Refactor schema/create-dimension!