tasksToExecutors uses grouping count as executor count.

While these two quantities are the same, this is a pure refactoring.

In a minute, it will be possible for there to be less groupings than executors (acknowledging the possibility of more requested executors than regions.)

It would be nice to more explicitly understand that these excess executors are doing nothing: this is the first step towards that.
Stop a IndexOutOfBounds exception when inflating makes empty groups.

Empty groups (in RegionMapping) represent executors doing nothing, and that's bad.  We'll try to get rid of those momentarily.