Isaac/snapshot api

Various fixes to snapshot API.

**Things fixed/added:**
- [x] Improve snapshot tests to actually verify if events are not being plotted when asked not to (it does not cover the case when there are no events in the time period, but it catches a bug we've been having with no event query request)
- [x] Fix bug in building no event query request (Issue #99)
- [x] by host query does not work (Issue #99) --> Not a bug (curly brackets were missing around `host`)