Ability to build ticket Packages i.e. "family" tickets

**Is your feature request related to a problem? Please describe.**
No problem

**Describe the solution you'd like**
Ba able to build a purchasable package that includes a combination of exiting tickets at a specified price.

i.e. you can purchase a "family" package for a fixed amount (likely less than the sum of the included tickets) and receive the tickets included in the package.

**Describe alternatives you've considered**
I have been creating a ticket called "family" and manually assigning tickets to the customers after purchase. This is problematic, not only for the increased labour but it means the number of tickets sold no longer matches the expected attendance.