Pivot Tool does way too much

[nickzoic]

It's hard to describe, and harder to understand.
Can it be split into separate Index, Pivot and Aggregate operations?

(Group By needs fixing up anyway)

[nickzoic]

OK so the first step was to clean up the Group By tool ...

that was kind of successful but:

• the squillions of little checkboxes are annoying.
• (in pandas) you can ask for a median but there's no way to ask for a N%-ile for example.
• you can join back to the original data to spread the grouped quantities out, but this is slightly annoying to do, this could become an option in the group by plugin:
  

[nickzoic]

pivot plugin stripped back, probably can just be pandas pivot_table but not sure about dask support (see #21) so leaving this open for the mo.

[nickzoic]

this is done for now, although there's some more thinking to do here wrt how much complexity each "tool" should have.
(see #18)