Allow multiple projects under DatasetType/project

[RobLBaker]

Currently, the element can only take a single child element of type . In reality, many datasets are collected in connection with more than one project. Please consider opening up the element to allow multiple child elements of the type so that data collected in connection with multiple projects can be accurately and completely documented.

[RobLBaker]

Oops - it looks like this was already suggested a few years ago as part of #361.