Get all item groups

Retrieves item groups that allow merchants to create and manage large groups of related items, also known as Item with variants.
For example: A merchant sells a T-shirt that is available in various sizes and colors. Each of the T-shirt variations is an item within the T-shirt item group. When an item group is created, it appears in the Register app as a single button, and tapping it displays a choice of variations for sale.
Before adding items to an item group, you need to create the:

1. Item group.
2. Attributes, such as size and color.
3. Options for each attribute, such as small and blue.
4. Individual items and include the item group ID to associate the items with the group.
5. Associate the options with an item.

• An item can only belong to an item group if the item group ID is entered when the item is created.
• An item can only be a member of a single item group, and once it is part of an item group, it can never be removed or moved to another item group; it can only be deleted.
  Note: The item number of the item group is automatically generated by the Clover server as a combination of the item group name and the names of all the options associated with that item. This item number is not editable. If the item group name or option name is changed, then the item names are automatically regenerated.