Alternative scheduling

[richfitz]

Fully exploring this will require changes in mcstate too.

At the moment we do a static schedule so that for 4 threads and 16 particles we'd send them to threads

1111222233334444

however this typically comes after a sorting of likelihood so it's very likely that group 1 has similar parameters to each other and group 4 has similar parameters to each other, but that group 1 and 4 might have quite different parameters. This minimises within-group variance in parameters (and likely time taken) but maximises between-group variance. Not ideal!

So if we did this as

1234123412341234

We'd reduce the between-group variance.