Using Merging Windows and/or Triggers without a downstream aggregation should fail

[kennknowles]

Both merging windows (such as sessions) and triggering only actually happen at an aggregation (GroupByKey). We should produce errors in any of these cases:

1. Merging window used without a downstream GroupByKey
2. Triggers used without a downstream GroupByKey
3. Window inspected after inserting a merging window and before the GroupByKey

Imported from Jira BEAM-184. Original Jira may contain additional context.
Reported by: bchambers.