Explicitly deleniate export types

[dogversioning]

Problem Statement
smart-on-fhir/cumulus-aggregator#133 - we have multiple types of exports now and the aggregator handles them the same way

Desired behavior
We want to generate exports with enough metadata that the aggregator can handle them appropriately without needing to track much external state. We should have sections of the manifest where you can say 'this file is flat/cube/period metadata'. For now, we can just append that to the file suffix, though if we get many more types of this kind of data we can consider something a bit more versatile

[dogversioning]

resolved via #314