[BROKEN] make repairs to plot.convergence;

There's just a bunch of annoying type errors here and there that I need to get over or work around
[UNTESTED] repair plot.convergence, finally
[FIXED?] It all runs! It's just very, very, very slow to do cumulate()

And there's a bug with plot.convergence: the plots are all ridiculous!
They look like they are sliced in the wrong direction.
Cleanups, and add that precumulation flag to cumulate() that I was thinking about
[BROKEN] Fiddle with definitions in an attempt to make it not unreadable to use cummean(), because cumulate() is unusably slow
checkpoint