Tweaked telescoping-trends & tests to check that coefficients are calc'd independently over time

The new test for `telescoping-trends` specifically checks that trends coefficients are calculated independently over time. That is, the trends stats for 2007-01-01 are the same no matter whether they were calculated as part of a larger telescoping-trends (e.g. `{:est-start "2005-12-19" :est-end "2007-01-01"}` operation or just for that period (`{:est-start "2007-01-01" :est-end "2007-01-01"}`). This just makes sure that the mechanics of `telescoping-trends` actually works as intended, that there's not some weird interaction between `est-start` and `est-end` and the telescoping application of the trends functions.