Fix ndarray qois in pce_analysis

[bartoszbosak]

There was a problem with _describe method in pce_analysis - it returned simple scalar values for percentiles, which was not properly handled outside, where ndarray is required. This change seems to solve this problem and hopefully does not have a negative impact elsewhere.

[djgroen]

I've rerouted this ticket from David to Wouter. @bartoszbosak I just wanted to check: you still would like to have this merged into master, right?

[bartoszbosak]

I've rerouted this ticket from David to Wouter. @bartoszbosak I just wanted to check: you still would like to have this merged into master, right?

Hi @djgroen, yes it would be nice to have it merged. I think that my proposition doesn't brake anything, but please double check :)

[DavidPCoster]

@wedeling -- this seems to be waiting for your approval ...

[bartoszbosak]

Hi @wedeling, could you check this PR if it can be merged? Thanks in advance.