Multiple parent DataTypes

[madprime]

Separating these suggested/planned features out so they don't block PR #1078

General thoughts:

• ~~the new uploadable boolean for DataTypes implies the type can't be a parent for other types~~
• also, we want an uploadable type to potentially flag more than one data within it

so the thoughts are (1) to disallow designating uploadable datatypes as parents, and (2) allowing them to have multiple parent types.

[madprime]

The first one was straightforward enough (and annoying to fix later if it's violated), so I've added it to #1078