Implemented `collapse` keyword argument to `Timeseries.series()` which
will collapse all of the data in the date range into a single result.
Fixed collapsing and condensing of gauge types by returning the last
value in a series rather than a list.
Implements joining multiple timeseries into a single result in both
`Timeseries.get()` and `Timeseries.series()`
Adding concurrency of joined series to the roadmap
Tests for joined timeseries
Fixed bug in joining fine-grained data in get()
Fixed bug in mongo series fetching which mutated a shared list
Fix for joining counter series
When condensing gauge data, the last non-null value is used, rather than the last value in the series independent of whether it was set.