Enhancements to tsplot

The main enhancement here is that `tsplot` can now accept a long-form DataFrame as  the input data, along with strings identifying the fields containing information about time, units, etc. Additionally, for both array and DataFrame inputs it can take data structured into conditions and plot each condition separately. This required a mild backwards-incompatible change to the calling syntax, but the workaround is easy. I also substantially overhauled the timeseries doc notebook. A couple of other unrelated small things that popped up got fixed in here too.