Update to new key separator pattern

This is based on the pattern outlined here: https://github.com/cbmi/avocado/issues/109
Add data_version to DataField(w/migration) and cache agg values
Replace data_modified(timestamp) with data_version and update references

data_version is an integer that is incremented when the
--update_data_version argument is supplied when using the avocado data
command.
Add data aggregation methods to default list cached by `avocado cache`
Add tests against the data_version updating from the data command
Fix migration to incorporate previous changes
Merge pull request #117 from cbmi/issue-109

Add caching to aggregated values in DataField