Koalas: Support multi-level columns in pivot_table

[garawalid]

This issue tracks DataFrame.pivot_table (#386)
The actual implementation can handle values as a list but it's disabled right now with this condition :

if isinstance(values, list) and len(values) > 1:
    raise NotImplementedError('Values as list of columns is not implemented yet.')

We need to implement multi-level columns before updating pivot_table

[ueshin]

Hi @garawalid,
We recently added a basic multi-index support in columns (#590).
Does that help you submit a PR?

[garawalid]

@ueshin thanks for notifying me!
I'll see what I can do.