Need warning when anonymous choices, anonymous choice branches

Many systems we want to integrate with Apache Daffodil do not support anonymous choices. They require the choice (aka union) to be named.

Furthermore, they require each choice branch to be a named element, not a sequence of multiple elements. 

In Daffodil, if a choice is not effectively the model-group of an element, we need a warning to that effect.

We say "not effectively the model-group" because preceding or surrounding sequences that contain only DFDL statements should be allowed, and hidden groups can be ignored as well. 

Similarly, if a choice branch does not contain a single element, we need a warning. 

A way to escalate such warnings to errors is needed (DAFFODIL-2735).