ENH: Make multiple root-to-tip-plots to help with debugging

[corneliusroemer]

Inspired by https://github.com/neherlab/treetime/issues/227

It could be helpful to output (multiple) root-to-tip-plots as they become available.

Right now, it appears that multiple root-to-tip analyses are done, but only one plot is added to outputs.

For example, when a clock-filter is run, this should be able to be accompanied by a plot.

When an error happens straight after this step, one could then look at this plot to see what happened. In issue https://github.com/neherlab/treetime/issues/227 no plot becomes available - despite the possibility of having such a plot at the moment the error happens.

Things that an early stage rtt helps with:

1. Check dates are parsed correctly
2. See what the earliest clock-filter regression is
3. See what gets filtered out by the earliest clock-filter

I don't know how often rtt-like analyses are done, but the first one seems to not be output as early as posible.