Move to a floating point tasks_per_cell

We can work with tasks_per_cell values less than 1, so allow that.

Make sure we report the maximum value found across all ranks, that is a better guide than the value from rank 0. Note we do report all the individual values when verbose > 1, and we now also report the total numbers of cells and tasks across the whole application. These numbers are only available after the first step as they need to be collected (avoids a reduce per step).

Documentation of the Scheduler parameters is started...